Fix the links in third_party/mdn/README.md (#219)

Author: ksw2000

third_party/mdn/README.md

@@ -1,12 +1,12 @@
 ## Contents
 
-The content of this directory (i.e., [mdn.json]) is parsed from the
+The content of this directory (i.e., [mdn.json](./mdn.json)) is parsed from the
 https://github.com/mdn/content.git repository and used to provide API
 documentation comments for the generated portion of package:web.
 
 ## License
 
-See [LICENSE] for the license this content is under.
+See [LICENSE](./LICENSE) for the license this content is under.
 
 ## Process