diff --git a/tool/number.go b/tool/number.go
index 8195b48..aa6d737 100644
--- a/tool/number.go
+++ b/tool/number.go
@@ -1,5 +1,4 @@
-// (Re-)number the .go content files according to the
-// order in index.txt.
+// (Re-)number the book source directories according to the index.
 
 package main