use macro for /usr/local/plan9
diff --git a/man/man1/cmp.1 b/man/man1/cmp.1
index 5023b6a..a3ab1c9 100644
--- a/man/man1/cmp.1
+++ b/man/man1/cmp.1
@@ -42,7 +42,7 @@
 .BR 0 ,
 octal; with anything else, decimal.
 .SH SOURCE
-.B /usr/local/plan9/src/cmd/cmp.c
+.B \*9/src/cmd/cmp.c
 .SH "SEE ALSO"
 .IR diff (1) 
 .SH DIAGNOSTICS