git-diff-index.txt: make --cached description a proper sentence

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

Documentation/git-diff-index.txt

@@ -27,7 +27,7 @@ include::diff-options.txt[]
 	The id of a tree object to diff against.
 
 --cached::
-	do not consider the on-disk file at all
+	Do not consider the on-disk file at all.
 
 -m::
 	By default, files recorded in the index but not checked