docs: Update installation.mdx (#1084)

It read "So an url," so I merely changed the an to a.

website/docs/installation.mdx

@@ -18,7 +18,7 @@ Immer can be installed as a direct dependency, and will work in any ES5 environm
 - CDN: Exposed global is `immer`
   - Unpkg: `<script src="https://unpkg.com/immer"></script>`
   - JSDelivr: `<script src="https://cdn.jsdelivr.net/npm/immer"></script>`
-  - ⚠️ When using a CDN, it is best to check the url in your browser and see what version it resolves to, so that your users aren't accidentally served a newer version in the future when updates are release. So use an url like: https://unpkg.com/[email protected]/dist/immer.umd.production.min.js instead. Substitute `production.min` with `development` in the URL for a development build.
+  - ⚠️ When using a CDN, it is best to check the url in your browser and see what version it resolves to, so that your users aren't accidentally served a newer version in the future when updates are release. So use a url like: https://unpkg.com/[email protected]/dist/immer.umd.production.min.js instead. Substitute `production.min` with `development` in the URL for a development build.
 
 ## Pick your Immer version