gitworkflows: fix grammar in 'Merge upwards' rule

Signed-off-by: Kyle Meyer <kyle@kyleam.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitworkflows.txt

@@ -107,7 +107,7 @@ the unstable branch into the stable one.  Hence the following:
 .Merge upwards
 [caption="Rule: "]
 =====================================
-Always commit your fixes to the oldest supported branch that require
+Always commit your fixes to the oldest supported branch that requires
 them.  Then (periodically) merge the integration branches upwards into each
 other.
 =====================================