docs: Remove outdated instruction from contribution guide (#4065)

The process of updating the Terraform module documentation is now run on
the `main` branch
([example](#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](#4063 (comment)),
I'm removing this reference, to prevent any confusion for new
contributors.

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