replace twitter with threadreaderapp in form_validations.md

Replace effectively dead Twitter link with threadreaderapp equivalent.
Since Twitter/X is moving towards a subscriber model and has started locking down the platform, unregistered visitors are unable to view comments to posts. This means this entire Twitter/X thread is invisible unless you have an account.
However, threadreaderapp.com provides the thread in a nicely formatted way.

intermediate_html_css/forms/form_validations.md

@@ -194,7 +194,7 @@ It's also worth noting client-side validations are not a silver bullet for ensur
 ### Additional resources
 
 - Check out [HTML5Pattern](https://www.html5pattern.com/) for a list of commonly used pattern regular expressions you may find helpful.
-- Look through [this Twitter thread](https://twitter.com/vponamariov/status/1400388896136040454) of the do's and don'ts for form validation UX.
+- Look through [this Twitter thread](https://threadreaderapp.com/thread/1400388896136040454.html) of the do's and don'ts for form validation UX.
 - Check out these [10 Guidelines](https://www.nngroup.com/articles/errors-forms-design-guidelines/) for form validation design.
 - [Learn Regex: A Beginner’s Guide](https://www.sitepoint.com/learn-regex/) is a great resource for understanding how regex patterns are built and shows how to use a fantastic tool for creating them. 
 - [Demystifying Regex with Practical Examples](https://www.sitepoint.com/demystifying-regex-with-practical-examples/) contains some practical examples in detail so you can understand how they work rather than simply copying and pasting.