Fix inappropriate comments in function ReplicationSlotAcquire

Hi,

In the function ReplicationSlotAcquire(), I found there is a missing in the
below comments:

```
/*
* Search for the slot with the specified name if the slot to acquire is
* not given. If the slot is not found, we either return -1 or error out.
*/
s = SearchNamedReplicationSlot(name, false);
if (s == NULL || !s->in_use)
{
LWLockRelease(ReplicationSlotControlLock);

ereport(ERROR,
(errcode(ERRCODE_UNDEFINED_OBJECT),
errmsg("replication slot \"%s\" does not exist",
name)));
}
```

It seems that when the slot is not found, we will only error out and will not
return -1. Attach the patch to remove the inappropriate description of return
value.

Regards,
Wang Wei