RelNotes 2.20: drop spurious double quote

We have three double-quote characters, which is one too many or too few.
Dropping the last one seems to match the original intention best.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.20.0.txt

@@ -578,7 +578,7 @@ Fixes since v2.19
  * "git rev-parse --exclude=* --branches --branches"  (i.e. first
    saying "add only things that do not match '*' out of all branches"
-   and then adding all branches, without any exclusion this time")
+   and then adding all branches, without any exclusion this time)
    worked as expected, but "--exclude=* --all --all" did not work the
    same way, which has been fixed.
    (merge 5221048092 ag/rev-parse-all-exclude-fix later to maint).