use macro for /usr/local/plan9
diff --git a/man/man3/exec.3 b/man/man3/exec.3
index 285af11..2c5b166 100644
--- a/man/man3/exec.3
+++ b/man/man3/exec.3
@@ -103,9 +103,9 @@
 (see
 .IR notify (3)).
 .SH SOURCE
-.B /usr/local/plan9/src/lib9/exec.c
+.B \*9/src/lib9/exec.c
 .br
-.B /usr/local/plan9/src/lib9/execl.c
+.B \*9/src/lib9/execl.c
 .SH SEE ALSO
 .IR prof (1),
 .IR intro (3),