ruby exercises: change directory names to follow name convention

[DennisNeu]

Because

The directories in "ruby exercises" don't match the naming convention. With this pull request, directory names are changed to match the convention ("1_data_types" -> "01_data_types"), thus putting folders into a nice, readable order and making navigating files easier.

This PR

• changes directory names to match naming convention (add a 0 infront of single digits)

Issue

Closes https://github.com/TheOdinProject/ruby-exercises/issues/101

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).