docs: Remove outdated instruction from contribution guide (#4065)

The process of updating the Terraform module documentation is now run on
the `main` branch
([example](https://github.com/philips-labs/terraform-aws-github-runner/pull/3919)),
so it no longer needs to be run on forked repositories (it also requires
credentials for the GitHub App).

As per @npalm's
[comment](https://github.com/philips-labs/terraform-aws-github-runner/pull/4063#issuecomment-2286734539),
I'm removing this reference, to prevent any confusion for new
contributors.

Author: janslow

CONTRIBUTING.md

@@ -71,7 +71,6 @@ Before you submit your pull request consider the following guidelines:
         terraform init
         ```
 
-* For updating docs, you have to enable GitHub actions on your forked repository. Simply go to the tab Actions and enable actions.
 * Commit your changes using a descriptive commit message:
 
     ```shell