Fix the path where footer is written. (#114469)

The footer file was being written in dev/bot instead of dev/docs.

Bug: https://github.com/flutter/flutter/issues/113805

dev/bots/post_process_docs.dart

@@ -65,7 +65,7 @@ Future<void> postProcess() async {
   final File versionFile = File('version');
   final String version = versionFile.readAsStringSync();
   // Recreate footer
-  final String publishPath = path.join(docsPath, 'doc', 'api', 'footer.js');
+  final String publishPath = path.join(docsPath, '..', 'docs', 'doc', 'flutter', 'footer.js');
   final File footerFile = File(publishPath)..createSync(recursive: true);
   createFooter(footerFile, version);
 }