Documentation/git-show-branch: work around "single quote" typesetting glitch

The displayed example is typeset with acute accents around the string that
should be surrounded by a pair of single quotes in manpage.  Replace them
with double quotes (the semantics of the example does not change).

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-branch.txt

@@ -172,7 +172,7 @@ only the primary branches.  In addition, if you happen to be on
 your topic branch, it is shown as well.
 
 ------------
-$ git show-branch --reflog='10,1 hour ago' --list master
+$ git show-branch --reflog="10,1 hour ago" --list master
 ------------
 
 shows 10 reflog entries going back from the tip as of 1 hour ago.