Fixes classname to match package and CSS rule

During the creation of block-editor the className on the autocompleter wasn’t updated. This change updates the class to use the `block-editor-` prefix. This causes the CSS to apply again thereby correcting the margin that was missing.

packages/editor/src/components/autocompleters/block.js

@@ -54,7 +54,7 @@ export function createBlockCompleter( {
 } = {} ) {
 	return {
 		name: 'blocks',
-		className: 'editor-autocompleters__block',
+		className: 'block-editor-autocompleters__block',
 		triggerPrefix: '/',
 		options() {
 			const selectedBlockName = getSelectedBlockName();