doc: Correct type of bgw_notify_pid

This has apparently been wrong since the beginning
(090d0f2).

Discussion: https://www.postgresql.org/message-id/289c2e45-c7d9-5ce4-7eff-a9e2a33e1580@enterprisedb.com

Author: petere

doc/src/sgml/bgworker.sgml

@@ -63,7 +63,7 @@ typedef struct BackgroundWorker
     char        bgw_function_name[BGW_MAXLEN];
     Datum       bgw_main_arg;
     char        bgw_extra[BGW_EXTRALEN];
-    int         bgw_notify_pid;
+    pid_t       bgw_notify_pid;
 } BackgroundWorker;
 </programlisting>
   </para>