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

Add description of new libmach.
diff --git a/man/man3/string.3 b/man/man3/string.3
index 93044a8..3ac364c 100644
--- a/man/man3/string.3
+++ b/man/man3/string.3
@@ -233,4 +233,4 @@
 .SH SOURCE
 .B /sys/src/libString
 .SH SEE ALSO
-.IR bio (2)
+.IR bio (3)