Fix: repoint link for problem solving

[DiZeroX]

Because

Site reorganization left a dead link. A previous PR fixed this issue for some links, but the one for Problem Solving was missed.

The url is mistakenly: https://www.theodinproject.com/guides/community/lessons/foundations-problem-solving
The correct url is: https://www.theodinproject.com/lessons/foundations-problem-solving

This PR

• fixes link for Problem Solving page

Issue

Related to #3842

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the keyword: brief description of change format, using one of the following keywords:
• Feature - adds new or amends existing user-facing behavior
• Chore - changes that have no user-facing value, refactors, dependency bumps, etc
• Fix - bug fixes
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• I have verified all tests and linters pass after making these changes.
• If this PR addresses an open issue, it is linked in the Issue section
• If applicable, this PR includes new or updated automated tests

[KevinMulhern]

Thanks for taking the time to fix this @DiZeroX, just a bit of feedback to simplify it.