Change subhead to say subheading

Try branch for #5790.

core-blocks/subhead/index.js

@@ -19,7 +19,7 @@ import './style.scss';
 export const name = 'core/subhead';
 
 export const settings = {
-	title: __( 'Subhead' ),
+	title: __( 'Subheading' ),
 
 	description: __( 'What\'s a subhead? Smaller than a headline, bigger than basic text.' ),