Fix comment in slot.c.

Reported-by: Sawada Masahiko
Author: Sawada Masahiko
Reviewed-by: Amit Kapila
Backpatch-through: 9.5
Discussion: https://postgr.es/m/CA+fd4k4Ws7M7YQ8PqSym5WB1y75dZeBTd1sZJUQdfe0KJQ-iSA@mail.gmail.com

src/backend/replication/slot.c

@@ -126,7 +126,7 @@ ReplicationSlotsShmemSize(void)
 }
 
 /*
- * Allocate and initialize walsender-related shared memory.
+ * Allocate and initialize shared memory for replication slots.
  */
 void
 ReplicationSlotsShmemInit(void)