README: fix link to CONTRIBUTING

Since the main branch is not called master anymore, and the anchor was
renamed, we fix both these changes in the link to that page from the
README.

README.md

@@ -42,7 +42,7 @@ key points.
 First, [download a pre-built Packer
 binary](https://www.packer.io/downloads.html) for your operating system or
 [compile Packer
-yourself](https://github.com/hashicorp/packer/blob/master/.github/CONTRIBUTING.md#setting-up-go-to-work-on-packer).
+yourself](https://github.com/hashicorp/packer/blob/main/.github/CONTRIBUTING.md#setting-up-go).
 
 After Packer is installed, create your first template, which tells Packer
 what platforms to build images for and how you want to build them. In our