refman/coqdoc.tex: fix two erroneous \url
refman/Omega.tex: do not advertize Pierre Cregut's email for bug reports
refman: remove ?uri=referer in urls pointing to validator.w3.org

 Unfortunately, these ?uri=referer parameters do not work correctly
 now that coq.inria.fr forces the switch to https before answering
 any document. See: http://validator.w3.org/docs/help.html#faq-referer
 I currently see no workaround for that, apart from generating links
 like ?uri=http://the.real.url/of/my/page, which would be quite painful.
 For now, users interested in checking the validity of our pages
 will have to copy-paste the url they want to check after clicking
 on the validator button.
refman: fix broken urls