ed(1): Add two references in the SEE ALSO section

Obtained from:	OpenBSD
MFC after:	1 week

bin/ed/ed.1

@@ -1,5 +1,5 @@
 .\" $FreeBSD$
-.Dd November 3, 2018
+.Dd April 9, 2021
 .Dt ED 1
 .Os
 .Sh NAME
@@ -961,6 +961,16 @@ USD:12-13
 .%O Addison-Wesley
 .%D 1981
 .Re
+.Rs
+.\" 4.4BSD USD:9
+.%A B. W. Kernighan
+.%T A Tutorial Introduction to the UNIX Text Editor
+.Re
+.Rs
+.\" 4.4BSD USD:10
+.%A B. W. Kernighan
+.%T Advanced Editing on UNIX
+.Re
 .Sh LIMITATIONS
 The
 .Nm