Update broken link. Will backport to 2.5.

Doc/lib/emailgenerator.tex

@@ -33,7 +33,7 @@ Optional \var{mangle_from_} is a flag that, when \code{True}, puts a
 line.  This is the only guaranteed portable way to avoid having such
 lines be mistaken for a \UNIX{} mailbox format envelope header separator (see
 \ulink{WHY THE CONTENT-LENGTH FORMAT IS BAD}
-{http://home.netscape.com/eng/mozilla/2.0/relnotes/demo/content-length.html}
+{http://www.jwz.org/doc/content-length.html}
 for details).  \var{mangle_from_} defaults to \code{True}, but you
 might want to set this to \code{False} if you are not writing \UNIX{}
 mailbox format files.