[ruby/did_you_mean] Fix `frozen_string_literal is ignored after any tokens` warning. (https://github.com/ruby/did_you_mean/pull/172)

```
did_you_mean/formatters/verbose_formatter.rb:5: warning: `frozen_string_literal' is ignored after any tokens
```

https://github.com/ruby/did_you_mean/commit/531760f323