Previously *.rkt files were handled by SchemeLexer. Although Racket is a descendant of Scheme, and that was better than nothing, it was not as good as it could be. For example, Racket treats square brackets  like parentheses (), but SchemeLexer highlights them as "errors".
I'm very sorry. I had deliberately included examples of errors in example.rkt to demonstrate that the lexer was correctly highlighting them as errors. I did not realize doing so would cause the unit tests to fail.
I pushed a new commit. I'll issue a new pull request.
Thank you very much for your patience with my contribution.