use macro for /usr/local/plan9
diff --git a/man/man3/mach-cmd.3 b/man/man3/mach-cmd.3
index 202ca39..ac6b3bb 100644
--- a/man/man3/mach-cmd.3
+++ b/man/man3/mach-cmd.3
@@ -131,7 +131,7 @@
 .I attachargs
 fills them in as best it can.
 .SH SOURCE
-.B /usr/local/plan9/src/libmach
+.B \*9/src/libmach
 .SH "SEE ALSO
 .IR mach (3),
 .IR mach-file (3),