docs: fix Common Issues typo

[Rohan5commit]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

AI usage:

• Used Codex to identify the duplicate word and prepare the one-line patch.
• Manually reviewed the surrounding Common Issues section to confirm the diff only changes wording.

Summary:

• Fixes a typo in docs/Common-Issues.md by changing in in to in.

Related issue:

• N/A

Guideline alignment:

• https://github.com/Homebrew/brew/blob/HEAD/CONTRIBUTING.md
• https://docs.brew.sh/How-To-Open-a-Homebrew-Pull-Request

Validation:

• git diff --check
• No tests added; wording-only change.