Fix argument in example

Thanks to Wei Xu
http://stacks.math.columbia.edu/tag/00F1#comment-741
Typo in topology.tex
Fix argument
Elucidate properties of zerodivisors in reduced rings

Thanks to Wei Xu
http://stacks.math.columbia.edu/tag/00H0#comment-744
http://stacks.math.columbia.edu/tag/030C#comment-745
Improve the proof of a lemma

Thanks to Keenan Kidwell
http://stacks.math.columbia.edu/tag/04KP#comment-749
http://stacks.math.columbia.edu/tag/037V#comment-748