use macro for /usr/local/plan9
diff --git a/man/man1/plumb.1 b/man/man1/plumb.1
index c296a1e..4885b90 100644
--- a/man/man1/plumb.1
+++ b/man/man1/plumb.1
@@ -85,7 +85,7 @@
 mount point for
 .IR plumber (4).
 .SH SOURCE
-.B /usr/local/plan9/src/cmd/plumb
+.B \*9/src/cmd/plumb
 .SH "SEE ALSO"
 .IR plumb (3),
 .IR plumber (4),