Revert "Experimental change for display on website"

This reverts commit c3ef9cec3e2b35cd851381da3cb9b6de203bc942.
Change section title to be more accurate
Fix an error in derived.tex

The proof of this very important lemma was wrong as we blithly used that
limits preserve exactness which is nonsense. Argh!

Thanks to Nuno Cardoso
http://stacks.math.columbia.edu/tag/05SC#comment-556