Update "Create your own recipe step function"

[EmilHvitfeldt]

This PR will close https://github.com/tidymodels/tidymodels.org/issues/34.

It furthers:

• removes the eval: false I put in there last time
• changes the name of the example step to step_percentiles() instead of step_percentile() which is the name of the implemented step. This way we don't run into this cycle
• updated code to match current best practices

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