trivial: Add missing period in documentation

Signed-off-by: Phil Hord <hordp@cisco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -480,7 +480,7 @@ endif::git-format-patch[]
 
 --ignore-submodules[=<when>]::
 	Ignore changes to submodules in the diff generation. <when> can be
-	either "none", "untracked", "dirty" or "all", which is the default
+	either "none", "untracked", "dirty" or "all", which is the default.
 	Using "none" will consider the submodule modified when it either contains
 	untracked or modified files or its HEAD differs from the commit recorded
 	in the superproject and can be used to override any settings of the