Fix README example for GET file size request

Corrects the example curl command in the README to match the correct endpoint for fetching file size. This ensures users can accurately follow documentation instructions.

README.md

@@ -97,7 +97,7 @@ response body: `base64 string`
 **GET `INSTANCE_URL/size/{download_key}`**: get the file size
 
 ```sh
-curl --request GET 'http://localhost:3000/expiration/db6186c8795740379d26fc61ecba1a24'
+curl --request GET 'http://localhost:3000/size/db6186c8795740379d26fc61ecba1a24'
 ```
 response: