Rewrite page(2) references to page(3).

Add description of new libmach.
diff --git a/man/man3/prime.3 b/man/man3/prime.3
index be9fabe..db22e5b 100644
--- a/man/man3/prime.3
+++ b/man/man3/prime.3
@@ -93,8 +93,8 @@
 .SH SOURCE
 .B /sys/src/libsec
 .SH SEE ALSO
-.IR aes (2)
-.IR blowfish (2),
-.IR des (2),
-.IR elgamal (2),
-.IR rsa (2),
+.IR aes (3)
+.IR blowfish (3),
+.IR des (3),
+.IR elgamal (3),
+.IR rsa (3),