Fix grammar and punctuation in chat agent mode documentation

[visma-zohaib-aslam]

This pull request includes several improvements to the documentation for using agent mode in Visual Studio Code. The changes primarily focus on improving clarity and readability by fixing grammatical issues and enhancing the flow of the text.

Improvements to documentation clarity and readability:

• Corrected grammatical errors and improved sentence structure for better readability in various sections of docs/copilot/chat/chat-agent-mode.md [1] [2] [3] [4] [5] [6] [7] [8].

[visma-zohaib-aslam]

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[ntrogh]

@visma-zohaib-aslam thanks for contributing to our docs.

[ntrogh]

@visma-zohaib-aslam Some other edits were done to this doc before your PR, can you resolve these merge conflicts?