update example of AST for TokenTypes.INTERFACE_DEF

[pbludov]

https://checkstyle.org/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#INTERFACE_DEF

needs to be extended to show example of AST.
Source to be updated: https://github.com/checkstyle/checkstyle/blob/master/src/main/java/com/puppycrawl/tools/checkstyle/api/TokenTypes.java

PLEASE READ for more details: https://github.com/checkstyle/checkstyle/wiki/Good-practice-on-how-to-update-AST-in-TokenTypes

[theanmolsharma]

I am working on this.