config.txt: Add missing colons after option name

gitcvs.usecrlfattr --> gitcvs.usecrlfattr::

This fixes an asciidoc markup issue.

Signed-off-by: Teemu Likonen <tlikonen@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -693,7 +693,7 @@ gitcvs.logfile::
 	Path to a log file where the CVS server interface well... logs
 	various stuff. See linkgit:git-cvsserver[1].
 
-gitcvs.usecrlfattr
+gitcvs.usecrlfattr::
 	If true, the server will look up the `crlf` attribute for
 	files to determine the '-k' modes to use. If `crlf` is set,
 	the '-k' mode will be left blank, so cvs clients will