Fix typo in `ClientSessionGroup` doc string #3203

Job	Run time
checks / pre-commit	31s
checks / readme-snippets	9s
checks / test (3.10, highest, windows-latest)	1m 28s
checks / test (3.10, lowest-direct, ubuntu-latest)	37s
checks / test (3.10, lowest-direct, windows-latest)	1m 19s
checks / test (3.11, highest, ubuntu-latest)	35s
checks / test (3.11, highest, windows-latest)	1m 43s
checks / test (3.10, highest, ubuntu-latest)	37s
checks / test (3.11, lowest-direct, ubuntu-latest)	34s
checks / test (3.12, lowest-direct, windows-latest)	1m 41s
checks / test (3.11, lowest-direct, windows-latest)	1m 48s
checks / test (3.12, lowest-direct, ubuntu-latest)	34s
checks / test (3.12, highest, ubuntu-latest)	36s
checks / test (3.13, highest, ubuntu-latest)	35s
checks / test (3.13, lowest-direct, ubuntu-latest)	36s
checks / test (3.13, lowest-direct, windows-latest)	1m 43s
checks / test (3.13, highest, windows-latest)	1m 27s
checks / test (3.12, highest, windows-latest)	1m 29s
	18m 2s

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