[Concurrency] fix documentation typos in TaskGroup.swift

[Changhyun-Kyle]

Summary

This PR fixes small documentation issues in TaskGroup.swift:

• Corrected a grammatical error:

  • "Awaiting on an empty group immediate returns nil" → "immediately returns nil"
  • This also maintains consistency with similar phrasing found on lines 367 and 654.

• Fixed a markdown syntax typo:

  • "It`s guaranteed" → "It's guaranteed"
  • Using an apostrophe instead of a backtick is more appropriate in this context.

Motivation

Improves clarity and readability of developer-facing documentation comments. This is a non-functional change that does not affect compilation or behavior.

Affected File

• stdlib / public / Concurrency / TaskGroup.swift

Notes

These corrections are minor but contribute to maintaining the quality and polish of the standard library documentation.

[ktoso]

Thank you for the fix up

[ktoso]

@swift-ci please smoke test