doc: remove unsupported parameter from patch-id

The patch is read from standard input and not from a parameter.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: asheiduk

Documentation/git-patch-id.txt

@@ -56,9 +56,6 @@ OPTIONS
 
 	This is the default.
 
-<patch>::
-	The diff to create the ID of.
-
 GIT
 ---
 Part of the linkgit:git[1] suite