fix snippet suggestions

add more details to snippet scenario to create a failing step
Looking at the [failed test](https://travis-ci.org/cucumber/cucumber/jobs/44906761) I think you need to also look for `* ` here, as that's another valid step keyword; also `But `. This feels pretty crude though, and it won't work for i18n features. I wonder what the old behaviour was?

I'll have a look.