Capitalize "SHOULD" in "Serving METADATA.json" section

Author: brettcannon

peps/pep-0819.rst

@@ -188,7 +188,7 @@ Repository API. The JSON encoded version of the package metadata may also be
 served, via the following modifications to the Simple Repository API:
 
 A new attribute ``data-dist-info-metadata-json`` may be added to anchor tags
-in the Simple API. This attribute should have a value containing the hash
+in the Simple API. This attribute SHOULD have a value containing the hash
 information for the ``METADATA.json`` file in the same format as
 ``data-dist-info-metadata``. If ``data-dist-info-metadata-json`` is present,
 the repository MUST serve the JSON encoded metadata file at the