Fix missing colon in example code in function header

[manoj-km24]

In Doc/library/bisect.rst , a code snippet seems to miss a colon ':' ,violating python syntax.

>>> def grade(score)
   ...     i = bisect([60, 70, 80, 90], score)
   ...     return "FDCBA"[i]
   ...

This PR addresses this issue and corrects the example with a colon.

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📚 Documentation preview 📚: https://cpython-previews--148839.org.readthedocs.build/