development(7): mention the Git mirror list in the manual page

MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D29234

(cherry picked from commit 8ef03ce)

share/man/man7/development.7

@@ -23,7 +23,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 11, 2021
+.Dd March 19, 2021
 .Dt DEVELOPMENT 7
 .Os
 .Sh NAME
@@ -67,9 +67,9 @@ The push URL is:
 .Pp
 .Lk ssh://git@gitrepo.FreeBSD.org/src.git
 .Pp
-There is also a public, read-only GitHub mirror at:
+There is also a list of public, read-only Git mirrors at:
 .Pp
-.Lk https://github.com/freebsd/freebsd-src
+.Lk https://docs.FreeBSD.org/en/books/handbook/mirrors/#_external_mirrors
 .Pp
 The
 .Ql main