chore: fix spelling (behaviour → behavior) in multiple comment files

[Harshrj53]

Summary

This PR fixes a small spelling inconsistency in several comment blocks across
the React codebase. React uses American English, so the British spelling
"behaviour" was updated to "behavior" in three files:

• packages/react-devtools-extensions/src/contentScripts/proxy.js
• packages/react-reconciler/src/ReactFiberCommitWork.js
• packages/react/src/ReactAct.js

These changes are limited to comments only and do not affect any runtime code.

How did you test this change?

• Verified using git grep that no other occurrences of "behaviour" exist.
• Confirmed that the files build and lint normally.
• Since this modification only affects comments, no functional behavior was changed.

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