Documentation/urls.txt: Use substitution to escape square brackets

Documentation/urls.txt: Use substitution to escape square brackets

This changes "[user@]" to use {startsb} and {endsb} to insert [ and ],
similar to how {caret} is used in git-rev-parse.txt.

[jc: Removed a well-intentioned comment that broke the final
 formatting from the original patch.  While we are at it,
 updated the paragraph that claims to be equivalent to the
 section that was updated earlier without making matching
 changes.]

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>