Updated free-courses-en.md

[pcodex]

Added a free course on Kotlin for beginners from Udemy

What does this PR do?

Add resource(s)

For resources

Description

Updated the Kotlin section with a free Udemy course on Kotlin for beginners

Why is this valuable (or not)?

This course is suitable for beginners and also covers OOP

How do we know it's really free?

The webpage labels it as a free course

For book lists, is it a book? For course lists, is it a course? etc.

Course

Checklist:

• [x] Read our contributing guidelines.
• [x] Search for duplicates.
• [x] Include author(s) and platform where appropriate.
• [x] Put lists in alphabetical order, correct spacing.
• [x] Add needed indications (PDF, access notes, under construction).
• [x] Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!