Fix libgit2 link (#2170)

* Fixed libgit2 link

* Update URL

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Co-authored-by: Brandon Ording <bording@gmail.com>

Author: suterma

README.md

@@ -3,7 +3,7 @@
 [![CI](https://github.com/libgit2/libgit2sharp/actions/workflows/ci.yml/badge.svg)](https://github.com/libgit2/libgit2sharp/actions/workflows/ci.yml) 
 [![NuGet version (LibGit2Sharp)](https://img.shields.io/nuget/v/LibGit2Sharp.svg)](https://www.nuget.org/packages/LibGit2Sharp/)
 
-**LibGit2Sharp brings all the might and speed of [libgit2](http://libgit2.github.com/), a native Git implementation, to the managed world of .NET**
+**LibGit2Sharp brings all the might and speed of [libgit2](https://libgit2.org/), a native Git implementation, to the managed world of .NET**
 
 ## Online resources