clippy: fix warning from `needless_doctest_main` #313

Job	Run time
Style/lint (windows-latest)	1m 5s
Style/lint (ubuntu-latest)	18s
Style/lint (macos-latest)	21s
Build/Test (macos-latest)	23s
Style/format	21s
Style/spellcheck	5s
Code Coverage (macos-latest)	36s
Code Coverage (ubuntu-latest)	29s
Code Coverage (windows-latest)	1m 16s
Build/Test (ubuntu-latest)	24s
Build/Test (windows-latest)	1m 1s
	6m 19s

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