fixes link to dotnet-buildtools-prereqs-docker manifest.json

docs/Helix.md

@@ -133,5 +133,5 @@ Goal is to balance cost/flakiness against having some coverage of supported dist
 ## Example of adding a new docker image to helix
 
 - Example PR: dotnet/dotnet-buildtools-prereqs-docker#398
-- Summary is to update [manifest.json](https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/master/manifest.json) with an entry for the new dockerfiles, and then add the docker files as well to dotnet-buildtools-prereqs-docker
+- Summary is to update [manifest.json](https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/main/manifest.json) with an entry for the new dockerfiles, and then add the docker files as well to dotnet-buildtools-prereqs-docker
 - The resulting new docker queue id will be found in: [image-info.dotnet-dotnet-buildtools-prereqs-docker-main.json](https://github.com/dotnet/versions/blob/main/build-info/docker/image-info.dotnet-dotnet-buildtools-prereqs-docker-main.json)