Correct spelling in installation guide reference (vercel#66484)

One-word 'checkout' is a noun or adjective that refers to the act of
taking items out of a store after paying for them. 2. Two-words 'check
out' is a verb that refers to request someone to look at something.
Corrected the spelling of "checkout" to "check out" in the documentation
to improve readability and accuracy. This change ensures proper usage of
the phrase when referring to the installation guide.

<!-- Thanks for opening a PR! Your contribution is much appreciated.
To make sure your PR is handled as smoothly as possible we request that
you follow the checklist sections below.
Choose the right checklist for the change(s) that you're making:

## For Contributors

### Improving Documentation

- Run `pnpm prettier-fix` to fix formatting issues before opening the
PR.
- Read the Docs Contribution Guide to ensure your contribution follows
the docs guidelines:
https://nextjs.org/docs/community/contribution-guide

### Adding or Updating Examples

- The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- Make sure the linting passes by running `pnpm build && pnpm lint`. See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md

### Fixing a bug

- Related issues linked using `fixes #number`
- Tests added. See:
https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs
- Errors have a helpful link attached, see
https://github.com/vercel/next.js/blob/canary/contributing.md

### Adding a feature

- Implements an existing feature request or RFC. Make sure the feature
request has been accepted for implementation before opening a PR. (A
discussion must be opened, see
https://github.com/vercel/next.js/discussions/new?category=ideas)
- Related issues/discussions are linked using `fixes #number`
- e2e tests added
(https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
- Documentation added
- Telemetry added. In case of a feature if it's used or not.
- Errors have a helpful link attached, see
https://github.com/vercel/next.js/blob/canary/contributing.md


## For Maintainers

- Minimal description (aim for explaining to someone not on the team to
understand the PR)
- When linking to a Slack thread, you might want to share details of the
conclusion
- Link both the Linear (Fixes NEXT-xxx) and the GitHub issues
- Add review comments if necessary to explain to the reviewer the logic
behind a change

### What?

### Why?

### How?

Closes NEXT-
Fixes #

-->

Co-authored-by: Sam Ko <[email protected]>

docs/index.mdx

@@ -32,7 +32,7 @@ On the left side of the screen, you'll find the docs navbar. The pages of the do
 
 On the right side of the screen, you'll see a table of contents that makes it easier to navigate between sections of a page. If you need to quickly find a page, you can use the search bar at the top, or the search shortcut (`Ctrl+K` or `Cmd+K`).
 
-To get started, checkout the [Installation](/docs/getting-started/installation) guide.
+To get started, check out the [Installation](/docs/getting-started/installation) guide.
 
 ## App Router vs Pages Router