docs: fix erroneous github_repository_file docs (integrations#1540)

- Updates the docs with the correct default set for the branch argument

Co-authored-by: Nick Floyd <139819+nickfloyd@users.noreply.github.com>
Co-authored-by: Keegan Campbell <me@kfcampbell.com>

website/docs/d/repository_file.html.markdown

@@ -31,8 +31,7 @@ The following arguments are supported:
 
 * `file` - (Required) The path of the file to manage.
 
-* `branch` - (Optional) Git branch (if omitted, the default repository branch is used, which is usually `main`)
-  The branch must already exist; it will not be created if it does not already exist.
+* `branch` - (Optional) Git branch. Defaults to `main`.
 
 ## Attributes Reference