Fix mistake in example in topology.tex

Thanks to Wei Xu
http://stacks.math.columbia.edu/tag/02HZ#comment-634
Incomplete proof in topology.tex

Thanks to Wei Xu
http://stacks.math.columbia.edu/tag/005F#comment-636
Typo in cohomology.tex

Thanks to Daniel Litt
http://stacks.math.columbia.edu/tag/08CL#comment-637