use macro for /usr/local/plan9
diff --git a/man/man3/disk.3 b/man/man3/disk.3
index 30e4440..bec50f2 100644
--- a/man/man3/disk.3
+++ b/man/man3/disk.3
@@ -163,7 +163,7 @@
 systems; the numbers bear little or no resemblance
 to reality.
 .SH SOURCE
-.B /usr/local/plan9/src/libdisk/disk.c
+.B \*9/src/libdisk/disk.c
 .SH SEE ALSO
 Plan 9's
 \fIfloppy\fR(3) and \fIsd\fR(3)