Remove stray quotes in --pretty and --format documentation

Quotes (for emphasis) are used in option explanations, not the
headings.

Signed-off-by: Štěpán Němec <stepnem@gmail.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -44,7 +44,7 @@ OPTIONS
 --email::
 	Show the email address of each author.
 
---format[='<format>']::
+--format[=<format>]::
 	Instead of the commit subject, use some other information to
 	describe each commit.  '<format>' can be any string accepted
 	by the `--format` option of 'git log', such as '{asterisk} [%h] %s'.

Documentation/pretty-options.txt

@@ -1,5 +1,5 @@
---pretty[='<format>']::
---format='<format>'::
+--pretty[=<format>]::
+--format=<format>::
 
 	Pretty-print the contents of the commit logs in a given format,
 	where '<format>' can be one of 'oneline', 'short', 'medium',