Fix link to Skylark reference.
Addresses GitHub issue "Skylark API page is not available."
RELNOTES: None.
@@ -10,7 +10,7 @@ title: Extensions - Overview
The page is an overview of Skylark, the language used in Bazel. This should be
enough to get you started, but you may be interested in a more complete
[specification of Skylark](spec.md). For a complete list of functions and
-types, please check the [API reference](skylark-overview.html).
+types, please check the [API reference](lib/skylark-overview.html).
## Syntax
