Fix a mistake in the docstring of `sem` (#917) #870

Job	Run time
Documentation	2m 14s
Julia 1.0 - ubuntu-latest - x64 - push	4m 47s
Julia 1.0 - ubuntu-latest - x86 - push	5m 31s
Julia 1.0 - macOS-latest - x64 - push	5m 2s
Julia 1.0 - windows-latest - x64 - push	8m 27s
Julia 1.0 - windows-latest - x86 - push	9m 22s
Julia 1 - ubuntu-latest - x64 - push	3m 51s
Julia 1 - ubuntu-latest - x86 - push	4m 24s
Julia 1 - macOS-latest - x64 - push	4m 2s
Julia 1 - windows-latest - x64 - push	5m 48s
Julia 1 - windows-latest - x86 - push	7m 1s
Julia nightly - ubuntu-latest - x64 - push	6m 8s
Julia nightly - ubuntu-latest - x86 - push	7m 23s
Julia nightly - macOS-latest - x64 - push	6m 6s
Julia nightly - windows-latest - x64 - push	6m 50s
Julia nightly - windows-latest - x86 - push	8m 5s
	1h 35m 1s

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