Update pdf-endpoint.mdx

added error copy

Author: kathayl

src/content/docs/browser-rendering/rest-api/pdf-endpoint.mdx

@@ -90,7 +90,7 @@ curl -X POST https://api.cloudflare.com/client/v4/accounts/<acccountID>/browser-
 ```
 
 :::note[Request size limits]
-The PDF endpoint accepts request bodies up to 50 MB. Requests larger than this will fail with `???`.
+The PDF endpoint accepts request bodies up to 50 MB. Requests larger than this will fail with `Error: request entity too large`.
 :::
 
 ## Advanced usage