Update _EMPTY_METADATA comment

Author: vstinner

Lib/dataclasses.py

@@ -191,7 +191,7 @@ class _KW_ONLY_TYPE:
 KW_ONLY = _KW_ONLY_TYPE()
 
 # Since most per-field metadata will be unused, create an empty
-# read-only proxy that can be shared among all fields.
+# read-only dictionary that can be shared among all fields.
 _EMPTY_METADATA = frozendict()
 
 # Markers for the various kinds of fields and pseudo-fields.