Fix regression in Spacer block (#7024)

* Fix regression in Spacer block

There was a typo in the rename of the drag handle and associated CSS class.

* Change classnames in the CSS not the JS

The JS followed guidelines.

core-blocks/spacer/editor.scss

@@ -1,14 +1,14 @@
 .editor-block-list__block[data-type="core/spacer"].is-selected .editor-block-list__block-edit {
 	background: $light-gray-200;
 
-	.block-spacer__resize-handler-top,
-	.block-spacer__resize-handler-bottom {
+	.core-blocks-spacer__resize-handler-top,
+	.core-blocks-spacer__resize-handler-bottom {
 		display: block;
 	}
 }
 
-.block-spacer__resize-handler-top,
-.block-spacer__resize-handler-bottom {
+.core-blocks-spacer__resize-handler-top,
+.core-blocks-spacer__resize-handler-bottom {
 	display: none;
 	border-radius: 50%;
 	border: 2px solid white;