Fix Third Party Notices link in README.md

Author: daviwil

README.md

@@ -37,5 +37,5 @@ send pull requests if you'd like to contribute.
 ## License
 
 This extension is [licensed under the MIT License](LICENSE).  Please see the
-[third-party notices](Third Party Notices.txt) file for details on the original
+[third-party notices](Third%20Party%20Notices.txt) file for details on the original
 source of the TextMate definition that we use.