git-clean.txt: do not claim we will delete files with -n/--dry-run

It appears that the wrong option got included in the list of what will
cause git-clean to actually take action.  Correct the list.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -35,7 +35,7 @@ OPTIONS
 --force::
 	If the Git configuration variable clean.requireForce is not set
 	to false, 'git clean' will refuse to delete files or directories
-	unless given -f, -n or -i. Git will refuse to delete directories
+	unless given -f or -i. Git will refuse to delete directories
 	with .git sub directory or file unless a second -f
 	is given.