git-diff(1): "--c" -> "--cc" typo fix

git diff does not take a --c option.
Signed-off-by: NJonathan Nieder <jrnieder@uchicago.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/diff-generate-patch.txt

@@ -96,7 +96,7 @@ index fabadb8,cc95eb0..4866510
 +
 or like this (when '--cc' option is used):
 
-       diff --c file
+       diff --cc file
 
 2.   It is followed by one or more extended header lines
      (this example shows a merge with two parents):