Documentation: use single-parameter --cacheinfo in example

The single-parameter form is described as the preferred way.  Separate
arguments are only supported for backward compatibility.  Update the
example to the recommended form.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -191,7 +191,7 @@ merging.
 To pretend you have a file with mode and sha1 at path, say:
 
 ----------------
-$ git update-index --cacheinfo mode sha1 path
+$ git update-index --cacheinfo <mode>,<sha1>,<path>
 ----------------
 
 '--info-only' is used to register files without placing them in the object