Update target branch

ImageSharp.NET-Summer-Hackfest.md

@@ -13,7 +13,7 @@ The mains goals for the session are to allow you to learn something new while co
 
 ### How to Contribute
 
-First, start by choosing what you'd like to do - you can have a look at [issues up-for-grabs label](https://github.com/SixLabors/ImageSharp/issues?q=is%3Aopen+is%3Aissue+label%3Aup-for-grabs+sort%3Areactions-%2B1-desc). (Feel free to suggest other features also though, your ideas are most welcome!). Then fork the repo, clone it locally and your are ready to develop. Finally, create a work-in-progress pull request to the master branch with the title `WIP-{Your issue number}`. Make sure that you have merged the latest changes and that the PR has a readable diff (please avoid reformatting documents, refactoring stuff, changing CRLF to LF or tabs to spaces etc.)
+First, start by choosing what you'd like to do - you can have a look at [issues up-for-grabs label](https://github.com/SixLabors/ImageSharp/issues?q=is%3Aopen+is%3Aissue+label%3Aup-for-grabs+sort%3Areactions-%2B1-desc). (Feel free to suggest other features also though, your ideas are most welcome!). Then fork the repo, clone it locally and your are ready to develop. Finally, create a work-in-progress pull request to the `beta-1` branch with the title `WIP-{Your issue number}`. Make sure that you have merged the latest changes and that the PR has a readable diff (please avoid reformatting documents, refactoring stuff, changing CRLF to LF or tabs to spaces etc.)
 
 ### Which are the things I can work on?