s/checkout-cache/checkout-index/g for Documentation/git-ls-files.txt

This updates last place where checkout-cache gets mentioned wrongly
for checkout-index.

Signed-off-by:  Kai Ruemmler <kai.ruemmler@gmx.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-ls-files.txt

@@ -50,7 +50,7 @@ OPTIONS
 
 -k|--killed::
 	Show files on the filesystem that need to be removed due
-	to file/directory conflicts for checkout-cache to
+	to file/directory conflicts for checkout-index to
 	succeed.
 
 -z::