use macro for /usr/local/plan9
diff --git a/man/man3/ioproc.3 b/man/man3/ioproc.3
index 5c3ebda..2d5bbd7 100644
--- a/man/man3/ioproc.3
+++ b/man/man3/ioproc.3
@@ -185,7 +185,7 @@
 }
 .EE
 .SH SOURCE
-.B /usr/local/plan9/src/libthread
+.B \*9/src/libthread
 .SH SEE ALSO
 .IR dial (3),
 .IR open (3),