Avoid HTML entities like   and ©

Use numerical entities instead for XHTML compatiblity.

src/ol/source/OSM.js

@@ -12,7 +12,7 @@ import XYZ from '../source/XYZ.js';
  * @type {string}
  * @api
  */
-export const ATTRIBUTION = '© ' +
+export const ATTRIBUTION = '© ' +
       '<a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> ' +
       'contributors.';