2 times R ---> S

Thanks to Fred Rohrer
http://stacks.math.columbia.edu/tag/01M3#comment-127
Fix typo and reference

Thanks to Fred Rohrer
http://stacks.math.columbia.edu/tag/01M4#comment-128
http://stacks.math.columbia.edu/tag/01M4#comment-129
Remove "above"

Thanks to Fred Rohrer
http://stacks.math.columbia.edu/tag/01M7#comment-130