fix: Correct grammar in a11y.instructions.md

[jeremykohn]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, or skill file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, or skill with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

This PR fixes certain grammatical errors in a11y.instructions.md:

• Change the contraction it's to the possessive its
• Change the repeated word open open to open
• Change it understandable to it is understandable

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New collection file.
• New skill file.
• Update to existing instruction, prompt, agent, collection or skill.
• Other (please specify): Grammar corrections

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Additional Notes

These grammar errors were found manually. Automatic grammar checks could be added to this project later, if they can be configured to avoid false positives.

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

This PR corrects four grammatical errors in the accessibility instructions file (a11y.instructions.md), improving the clarity and professionalism of the documentation.

Changes:

• Fixed possessive form: changed "it's" to "its" in two locations (lines 55 and 116)
• Removed duplicate word: changed "open open" to "open" (line 94)
• Added missing verb: changed "it understandable" to "it is understandable" (line 197)