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

Denton-L · gitster · commit b277b737d987 · 2020-09-17T09:38:46.000-07:00
Signed-off-by: Denton Liu &lt;liu.denton@gmail.com&gt;
Signed-off-by: Junio C Hamano &lt;gitster@pobox.com&gt;
diff --git a/Documentation/git-diff-index.txt b/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
