diff-options.txt: Fix asciidoc markup issue

Must be "--patience::", not "--patience:".

Signed-off-by: Teemu Likonen <tlikonen@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -36,7 +36,7 @@ endif::git-format-patch[]
 --patch-with-raw::
 	Synonym for "-p --raw".
 
---patience:
+--patience::
 	Generate a diff using the "patience diff" algorithm.
 
 --stat[=width[,name-width]]::