doc: format.notes specify a ref under refs/notes/ hierarchy

There is no 'ref/notes/' hierarchy.  '[format] notes = foo' uses notes
that are found in 'refs/notes/foo'.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/format.txt

@@ -119,7 +119,7 @@ format.notes::
 	`--notes=<ref>`, where `ref` is the non-boolean value. Defaults
 	to false.
 +
-If one wishes to use the ref `ref/notes/true`, please use that literal
+If one wishes to use the ref `refs/notes/true`, please use that literal
 instead.
 +
 This configuration can be specified multiple times in order to allow