Fix github_repository_file import documentation

[winterjung]

Attach missing _file suffix

Without _file suffix, below error message is occurred.

$ terraform import github_repository.gitignore example/.gitignore 
Error: resource address "github_repository.gitignore" does not exist in the configuration.

Before importing this resource, please create its configuration in the root module. For example:

resource "github_repository" "gitignore" {
  # (resource arguments)
}

After fixing

$ terraform import github_repository_file.gitignore example/.gitignore
github_repository_file.gitignore: Importing from ID "example/.gitignore"...
github_repository_file.gitignore: Import prepared!
  Prepared github_repository_file for import
github_repository_file.gitignore: Refreshing state... [id=example/.gitignore]

Import successful!

The resources that were imported are shown above. These resources are now in
your Terraform state and will henceforth be managed by Terraform.

[anGie44]

LGTM, thanks @jungwinter! 🚀