documentation: Add missing word "ref"

In the git-fetch documentation, description of <refspec> syntax is not
entirely clear. When explaining about "the destination ref <dst>", word
"ref" is included. Logically, it should be the same in the explanation
of "the source <src>".

Signed-off-by: Monika Kairaityte <[email protected]>

Documentation/pull-fetch-param.txt

@@ -24,7 +24,7 @@ ifdef::git-pull[]
 endif::git-pull[]
 +
 The format of a <refspec> parameter is an optional plus
-`+`, followed by the source <src>, followed
+`+`, 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.  <src> is
 typically a ref, but it can also be a fully spelled hex object