tcp(4): fix a misnomer

This is almost certainly not the meaning of PCB used here.

Reviewed by:	markj
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D40786

share/man/man4/tcp.4

@@ -34,7 +34,7 @@
 .\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd June 6, 2023
+.Dd June 29, 2023
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -776,7 +776,7 @@ which both endpoints are local.
 .It Va path_mtu_discovery
 Enable Path MTU Discovery.
 .It Va pcbcount
-Number of active process control blocks
+Number of active protocol control blocks
 (read-only).
 .It Va perconn_stats_enable
 Controls the default collection of statistics for all connections using the