use macro for /usr/local/plan9
diff --git a/man/man1/factor.1 b/man/man1/factor.1
index 2ce1379..597484b 100644
--- a/man/man1/factor.1
+++ b/man/man1/factor.1
@@ -61,6 +61,6 @@
 is missing, it reads the starting number from the
 standard input.
 .SH SOURCE
-.B /usr/local/plan9/src/cmd/factor.c
+.B \*9/src/cmd/factor.c
 .br
-.B /usr/local/plan9/src/cmd/primes.c
+.B \*9/src/cmd/primes.c