Documentation/git-show-branch: avoid literal {apostrophe}

The {apostrophe} was needed at the time of a521845800 ("Documentation:
remove stray backslash in show-branch discussion", 2010-08-20).  All
other uses of {apostrophe} were removed in 6cf378f0cb ("docs: stop using
asciidoc no-inline-literal", 2012-04-26).

Unfortunately, the {apostrophe} is rendered literally with Asciidoctor
(at least with 1.5.5-2.0.3).  Avoid this by using single-quotes.

Escaping the leading single-quote allows the content to render properly
in AsciiDoc and Asciidoctor.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-branch.txt

@@ -167,7 +167,7 @@ $ git show-branch master fixes mhf
 ------------------------------------------------
 
 These three branches all forked from a common commit, [master],
-whose commit message is "Add {apostrophe}git show-branch{apostrophe}".
+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".