Fix an article usage

redis.conf

@@ -818,7 +818,7 @@ lua-time-limit 5000
 # A slave of a failing master will avoid to start a failover if its data
 # looks too old.
 #
-# There is no simple way for a slave to actually have a exact measure of
+# There is no simple way for a slave to actually have an exact measure of
 # its "data age", so the following two checks are performed:
 #
 # 1) If there are multiple slaves able to failover, they exchange messages