docs: remove stray backtick in contributing guide

[auyua9]

Summary

• remove the stray trailing backtick from the opening paragraph of the contributing guide
• keep the change scoped to the existing docs typo fix with no behavior changes

Testing

• not run (docs-only change)

[cursor]

PR Summary

Low Risk
Low risk docs-only typo fix with no runtime or behavior impact.

Overview
Removes a stray trailing backtick from the opening paragraph in docs/src/contributing/CONTRIBUTING.md to fix markdown formatting.

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