Fix typo in repl-ping-replica-period comment in redis.conf (redis#13782)

The comment for the `repl-ping-replica-period` option in `redis.conf`
mistakenly refers to `repl_ping_replica_period` (with underscores).
This PR corrects it to use the proper format with dashes, as per the
actual configuration option.

redis.conf

@@ -668,7 +668,7 @@ repl-diskless-sync-max-replicas 0
 repl-diskless-load disabled
 
 # Master send PINGs to its replicas in a predefined interval. It's possible to
-# change this interval with the repl_ping_replica_period option. The default
+# change this interval with the repl-ping-replica-period option. The default
 # value is 10 seconds.
 #
 # repl-ping-replica-period 10