Fix idiotic mistake in remark on points of fppf site

The mistake was to think that the stalk of an inverse limit is the
inverse limit of the stalks.

Also put in a reference to the preprint by Schroeer who constructs
some stalk functors which are (I think) not of the form described
in the remark.
Stack project corrections by Albert Gunawan

Thanks to Albert Gunawan
Fix reference

Caught by the script stacks-tools/checks/bib.py