doc: fix update-ref symref-create formatting

`symref-create` should be followed `::`, not `:`. The lack of second
colon (`:`) causes it to appear as regular text (`<p>`) instead of as a
description list term (`<dt>`) in the HTML documentation.

Signed-off-by: Sam Bostock <sam@sambostock.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sambostock

Documentation/git-update-ref.txt

@@ -109,7 +109,7 @@ verify::
 	Verify <ref> against <old-oid> but do not change it.  If
 	<old-oid> is zero or missing, the ref must not exist.
 
-symref-create:
+symref-create::
 	Create symbolic ref <ref> with <new-target> after verifying that
 	it does not exist.