Update fuction usage to reference scikit-learn instead of pd.get_dummies

[bkp5190]

This PR fixes a bug in the docs for validation curves where it references pd.get_dummies for one-hot encoding, but scikit-learn's OneHotEncoder function is used instead.

I have made the following changes:

1. Update the documentation to show the correct function being used for one-hot encoding

Questions for the @DistrictDataLabs/team-oz-maintainers:

• Do the docs still convey the same message

CHECKLIST

• Is the commit message formatted correctly?
• Have you noted the new functionality/bugfix in the release notes of the next release?

• Have you built the docs using make html (must be run from docs/)?
  See below for the updated location render

[bkp5190]