Documentation: grep: fix asciidoc problem with --

Asciidoc interprets two dashes separated by spaces as a single big
dash. So let's escape the first dash, so that "\--" will properly
appear as "--".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -183,7 +183,7 @@ OPTIONS
 Examples
 --------
 
-git grep 'time_t' -- '*.[ch]'::
+git grep 'time_t' \-- '*.[ch]'::
 	Looks for `time_t` in all tracked .c and .h files in the working
 	directory and its subdirectories.