storage_bucket: Correct website docs (#3235)

The website block of storage_bucket requires at least one of not_found_page and main_page_suffix to be set

Co-authored-by: Calle Pettersson <[email protected]>

third_party/terraform/website/docs/r/storage_bucket.html.markdown

@@ -131,7 +131,7 @@ The `versioning` block supports:
 
 * `enabled` - (Required) While set to `true`, versioning is fully enabled for this bucket.
 
-The `website` block supports:
+The `website` block supports the following elements, and requires at least one to be defined:
 
 * `main_page_suffix` - (Optional) Behaves as the bucket's directory index where
     missing objects are treated as potential directories.