Documentation: remove stray backslash in show-branch discussion

Because there is no unescaped apostrophe to pair it with, asciidoc
does not consider this apostrophe a candidate for escaping and
the backslash passes through.

Reported-by: Frédéric Brière <fbriere@fbriere.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-branch.txt

@@ -168,10 +168,10 @@ $ git show-branch master fixes mhf
 ------------------------------------------------
 
 These three branches all forked from a common commit, [master],
-whose commit message is "Add \'git show-branch\'". The "fixes"
-branch adds one commit "Introduce "reset type" flag to "git reset"".
-The "mhf" branch adds many other commits. The current branch
-is "master".
+whose commit message is "Add {apostrophe}git show-branch{apostrophe}".
+The "fixes" branch adds one commit "Introduce "reset type" flag to
+"git reset"". The "mhf" branch adds many other commits.
+The current branch is "master".
 
 
 EXAMPLE