Specify code block syntax

[dnskr]

Description

The PR specifies shell script syntax in the code blocks.

Motivation and Context

Better syntax highlighting and the ability to execute code from the IDE.

Impact

No

Test Plan

Compare how presto-docs/README.md rendered before and after changes.

Before - there are no launch buttons:

After - there are launch buttons:

Contributor checklist

• Please make sure your submission complies with our contributing guide, in particular code style and commit standards.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==

[steveburnett]

LGTM! (docs)