Fix change detection in tools/maint/update_docs.py. NFC (#25830)

Author: sbc100

tools/maint/update_docs.py

@@ -55,7 +55,7 @@ def main(args):
   # Build and install the docs
   subprocess.check_call(['make', 'install', f'EMSCRIPTEN_SITE={site_out}'], cwd=site_dir)
 
-  files_changed = get_changed_files(site_dir)
+  files_changed = get_changed_files(site_out)
   # This AUTHORS.html file happens to always contains the current date, so we don't want
   # to consider updates that contain only this one file
   if 'docs/contributing/AUTHORS.html' in files_changed: