vi(1): Add URL to the vi/ex reference manual in the man page

Reported in PR 241985

The manual page references the "vi/ex reference manual" but there is no
information about where to find that document. Add a reference to the manual in
the SEE ALSO section since the Project hosts a copy of it[1].

Change sent upstream[2]

If D26158 gets reviewed and committed, we could close that PR.

[1] https://docs.freebsd.org/44doc/usd/13.viref/paper.pdf).
[2] https://github.com/lichray/nvi2/pull/85

Submitted by:	freebsd@tim.thechases.com
Approved by:	manpages (gbe@)
Differential Revision:	https://reviews.freebsd.org/D26163

contrib/nvi/man/vi.1

@@ -12,7 +12,7 @@
 .\" that you would have purchased it, or if any company wishes to
 .\" redistribute it, contributions to the authors would be appreciated.
 .\"
-.Dd November 2, 2013
+.Dd September 25, 2020
 .Dt VI 1
 .Os
 .Sh NAME
@@ -2743,6 +2743,10 @@ and \*(Gt0 if an error occurs.
 .Xr ctags 1 ,
 .Xr iconv 1 ,
 .Xr re_format 7
+.Rs
+.%T vi/ex reference manual
+.%U https://docs.freebsd.org/44doc/usd/13.viref/paper.pdf
+.Re
 .Sh STANDARDS
 .Nm nex Ns / Ns Nm nvi
 is close to