pretty-options.txt: match --format's documentation with implementation.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-options.txt

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