use macro for /usr/local/plan9
diff --git a/man/man3/dup.3 b/man/man3/dup.3
index 3c7312d..a9378dc 100644
--- a/man/man3/dup.3
+++ b/man/man3/dup.3
@@ -26,7 +26,7 @@
 (closing any old file associated with
 .IR newfd ).
 .SH SOURCE
-.B /usr/local/plan9/src/lib9/dup.c
+.B \*9/src/lib9/dup.c
 .SH DIAGNOSTICS
 Sets
 .IR errstr .