Fix TOML URL in doc.go

doc.go

diff --git a/doc.go b/doc.go
index a231199..23c125a 100644
--- a/doc.go
+++ b/doc.go
@@ -1,7 +1,7 @@
 // Package toml is a TOML markup language parser.
 //
 // This version supports the specification as described in
-// https://github.com/toml-lang/toml/blob/master/versions/toml-v0.2.0.md
+// https://github.com/toml-lang/toml/blob/master/versions/en/toml-v0.4.0.md
 //
 // TOML Parsing
 //