docs(s3): add missing labels and fix wording

Co-authored-by: Zaiming (Stone) Shi <zmstone@gmail.com>

rel/i18n/emqx_s3_schema.hocon

@@ -9,8 +9,14 @@ secret_access_key.desc:
 bucket.desc:
 """The name of the S3 bucket."""
 
+bucket.label:
+"""Bucket"""
+
 key.desc:
-"""Key of the S3 object being manipulated."""
+"""Key of the S3 object."""
+
+key.label:
+"""Object Key"""
 
 host.desc:
 """The host of the S3 endpoint."""