Issue: #11838 Spring Boot - Unavailable URL

[migliusss]

What does this PR do?

Change a Spring Boot book resource to a valid URL.

For resources

Description

I have changed an unavailable URL to a valid URL in a Spring Boot book resource called Building modern Web Apps with Spring Boot and Vaadin.

Why is this valuable (or not)?

There will be a valid URL for a resource.

How do we know it's really free?

It is documentation for Vaadin and it is open source.

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

Yes it is a book(ish) in documentation format.

Checklist:

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

Follow-up

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

[eshellman]

Thanks, and welcome to free-programming-books!