Remove Ruby compile warning in internal plugin helper

What does this MR do and why?

This MR removes the following Ruby compile warnings:

$ ruby -wc spec/danger/plugins/internal/helper_spec.rb
spec/danger/plugins/internal/helper_spec.rb:13: warning: character class has duplicated range: /\A[added]/
spec/danger/plugins/internal/helper_spec.rb:13: warning: character class has duplicated range: /\A[modified]/

MR acceptance checklist

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