gh-101100: Fix Sphinx nits in `library/contextlib.rst` #96064

Job	Run time
Check for source changes	9s
Check if generated files are up to date	0s
macOS	0s
Windows	0s
Windows (free-threaded)	0s
macOS (free-threaded)	0s
Ubuntu	0s
Ubuntu (free-threaded)	0s
Address sanitizer	0s
Ubuntu SSL tests with OpenSSL	0s
CIFuzz	0s
Hypothesis tests on Ubuntu	0s
Docs / Docs	2m 23s
Docs / Docs (Oldest Sphinx)	1m 56s
Docs / Doctest	8m 52s
All required checks pass	4s
	13m 24s

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