Data types exercise: Fix typo in file name, strings_exercises_spec.rb → string_exercises_spec.rb

[gregorgatej]

Because

Filenames for the string exercises inside '1_data_types' folder aren't named the same across the 'exercises' and 'spec' folders . The fix changes this (to be also consistent with the description in steps '2.' and '3.' under 'Usage' in README).

This PR

• Renamed ruby_basics/1_data_types/spec/strings_exercises_spec.rb to ruby_basics/1_data_types/spec/string_exercises_spec.rb
• Renamed solutions/ruby_basics/1_data_types/spec/strings_exercises_spec.rb to solutions/ruby_basics/1_data_types/spec/string_exercises_spec.rb

Issue

Related to #101

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes changes to exercises, they are also changed in the solutions folder.