Exercise `hash_exercises.rb`: rename `favorite_list` to `favorite_hash` for clarity

[sometimescasey]

Because

favorite_hash is a less ambiguous parameter name

This PR

• renames all instances of favorite_list to favorite_hash in hash_exercises.rb
• and analogously, renames original_list and additional_list

Issue

Closes #96

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If this PR includes changes that needs to be updated on the solutions branch, I have created another PR (and linked it to this PR).