add missing .html suffix to documentation article link (#501)

Author: alexandersandberg

_data/documentation.yaml

@@ -45,7 +45,7 @@
       Swift has support for bidirectional interoperability with C++.
       A great variety of C++ APIs can be called directly from Swift, and select Swift APIs can be used from C++.
   - title: Value and Reference types
-    url: /documentation/articles/value-and-reference-types
+    url: /documentation/articles/value-and-reference-types.html
     description: |
       Types in Swift are grouped in two categories: value types and reference types.
       Each behave differently and understanding the difference is an important part of understanding Swift.