fix CSS link in README

[kens-visuals]

FIX: 30-seconds-of-css link in README

Description

30-seconds-of-css link in README.md file was broken, so I replaced it with a correct one.

What does your PR belong to?

• Questions / Answers
• Website
• General / Things regarding the repository (like CI Integration)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking improvement of a question)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have checked that the changes are working properly
• I have checked that there isn't any PR doing the same
• I have read the CONTRIBUTING document.

[fejes713]

Thanks for this catch @kens-visuals 👏