remove errant backslash in XRDS preamble.

This bit me when I was trying to get Extension:OpenID to work with
OpenID auth on my Gerrit installation.  Since removing this backslash
made it work, I'm assuming that it may fix other bugs elsewhere too.

It is supposed to be a typo, dating back earlier than 2008
(https://gerrit.wikimedia.org/r/gitweb?p=mediawiki/extensions/OpenID.git;a=blob;f=SpecialOpenIDXRDS.body.php;hb=418d1aa02730c2beb037a8bd90d971c830a291c7)

Change-Id: Ida19d8f554b7ce7f5ed2b979cc6221f1de7217e4