pull doc: fix a long-standing grammar error

It should be "is not an empty string" not "is not empty string". This
fixes wording originally introduced in ab9b31386b ("Documentation:
multi-head fetch.", 2005-08-24).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pull-fetch-param.txt

@@ -33,7 +33,7 @@ name.
 it requests fetching everything up to the given tag.
 +
 The remote ref that matches <src>
-is fetched, and if <dst> is not empty string, the local
+is fetched, and if <dst> is not an empty string, the local
 ref that matches it is fast-forwarded using <src>.
 If the optional plus `+` is used, the local ref
 is updated even if it does not result in a fast-forward