Updated a broken link + added missing name credit for a title (#5757)

* Updated a broken llink

Updated a broken link to the book "Mastering Xamarin UI Development, Second Edition" by Steven F. Daniel

* Added missing author to title

Added missing author name to the title "Creating Mobile Apps with Xamarin.Forms C#" by Charles Petzold

books/free-programming-books-langs.md

@@ -449,7 +449,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [C# Programming Yellow Book](https://www.robmiles.com/s/CSharp-Book-2019-Refresh.pdf) - Rob Miles (PDF) (2019)
 * [C# Smorgasbord](https://www.filipekberg.se) - Filip Ekberg (HTML) [(PDF, EPUB, MOBI)](https://www.filipekberg.se/2018/04/02/csharp-smorgasbord-free/) (2018)
 * [C# Tips](https://leanpub.com/cstips) - Jason Roberts *(Leanpub account or valid email requested)*
-* [Creating Mobile Apps with Xamarin.Forms C#](https://developer.xamarin.com/guides/xamarin-forms/creating-mobile-apps-xamarin-forms/)
+* [Creating Mobile Apps with Xamarin.Forms C#](https://developer.xamarin.com/guides/xamarin-forms/creating-mobile-apps-xamarin-forms/) - Charles Petzold
 * [Daily Design Patterns](https://web.archive.org/web/20170930132000/https://www.exceptionnotfound.net/downloads/dailydesignpattern.pdf) - Matthew P Jones (PDF)
 * [Data Structures and Algorithms with Object-Oriented Design Patterns in C#](https://web.archive.org/web/20161220072449/http://www.brpreiss.com/books/opus6/) - Bruno Preiss
 * [Dissecting a C# Application](https://damieng.com/blog/2007/11/08/dissecting-a-c-application-inside-sharpdevelop) - Christian Holm, Bernhard Spuida, Mike Kruger
@@ -457,7 +457,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [High level asynchronous programming with Reactive Extensions](https://github.com/petroemil/Rx.Book) - Emil Petro
 * [Introduction to Rx](http://www.introtorx.com)
 * [Learn C# in Y Minutes](https://learnxinyminutes.com/docs/csharp/)
-* [Mastering Xamarin UI Development, Second Edition](https://www.packtpub.com/free-ebooks/mastering-xamarin-ui-development-second-edition) - Steven F. Daniel (Packt account *required*)
+* [Mastering Xamarin UI Development, Second Edition](https://www.packtpub.com/free-ebook/mastering-xamarin-ui-development-second-edition/9781788995511) - Steven F. Daniel (Packt account *required*)
 * [.NET Book Zero](http://www.charlespetzold.com/dotnet) - Charles Petzold (PDF, XPS)
 * [Object Oriented Programming using C#](http://bookboon.com/en/object-oriented-programming-using-c-sharp-ebook) - Simon Kendal, Bookboon. (email address *requested*, not required)
 * [Threading in C#](http://www.albahari.com/threading/)