Fix typo in proc.c

Author: Julien Rouhaud
Discussion: https://postgr.es/m/20200206082333.GA95343@nol

src/backend/storage/lmgr/proc.c

@@ -851,7 +851,7 @@ ProcKill(int code, Datum arg)
 
 	/*
 	 * Detach from any lock group of which we are a member.  If the leader
-	 * exist before all other group members, it's PGPROC will remain allocated
+	 * exist before all other group members, its PGPROC will remain allocated
 	 * until the last group process exits; that process must return the
 	 * leader's PGPROC to the appropriate list.
 	 */