fetch doc: update refspec format description

The text made it sound as if the leading plus is the only thing that
is optional, and forgot that <lhs> is the same as <lhs>:, i.e. fetch
it and do not store anywhere.

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

Documentation/pull-fetch-param.txt

@@ -15,6 +15,7 @@ endif::git-pull[]
 	The format of a <refspec> parameter is an optional plus
 	`+`, followed by the source ref <src>, followed
 	by a colon `:`, followed by the destination ref <dst>.
+	The colon can be omitted when <dst> is empty.
 +
 The remote ref that matches <src>
 is fetched, and if <dst> is not empty string, the local