Mention both authoring locations in the block placeholder

The simplified summary can be written in the editor sidebar or the
Accessibility Checker meta box Readability tab, so the placeholder now
points to both instead of only the sidebar.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: SteveJonesDev

src/simplifiedSummaryBlock/edit.js

@@ -26,7 +26,7 @@ const Edit = () => {
 			<h2>{ heading }</h2>
 			<p className="edac-simplified-summary-block__placeholder">
 				{ __(
-					'The simplified summary for this post will display here. Write the summary in the Accessibility Checker sidebar panel.',
+					'The simplified summary for this post will display here. Write the summary in the Readability section of the Accessibility Checker sidebar or meta box.',
 					'accessibility-checker'
 				) }
 			</p>