Fix mistake in docs error return values (GH-4876)
Fix mistake in docs error return values (GH-4876)

Fixes https://github.com/cython/cython/issues/4875
da-woods/cython,cython/cython,cython/cython,da-woods/cython,cython/cython,scoder/cython,da-woods/cython,scoder/cython,scoder/cython,cython/cython,da-woods/cython,scoder/cython