Fix link label in the documentation for contextvars.ContextVar

[ahryniv]

Summary

Fixing a typo in the documentation page: https://www.starlette.io/middleware/#limitations

The title for a link https://docs.python.org/3/library/contextvars.html#contextvars.ContextVar is not correct.

Current value: contextlib.ContextVar
Fixing to: contextvars.ContextVar

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

Thanks!