use macro for /usr/local/plan9
diff --git a/man/man3/proto.3 b/man/man3/proto.3
index ab8d304..fcd3904 100644
--- a/man/man3/proto.3
+++ b/man/man3/proto.3
@@ -125,7 +125,7 @@
 .B /sys/lib/sysconfig/proto/portproto
 generic prototype file.
 .SH SOURCE
-.B /usr/local/plan9/src/libdisk/proto.c
+.B \*9/src/libdisk/proto.c
 .SH SEE ALSO
 .IR mk9660 (8),
 Plan 9's \fImkfs\fR(8)