Fix README.html links

Author: droserasprout

docs/mdbook-summary

@@ -66,7 +66,7 @@ def _preprocess_content(text: str, links: dict[str, str], depth: int) -> str:
             title += f' → {anchor}'
             anchor = f'#{anchor}'
 
-        url = f'[{title}]({full_path}{anchor})'.replace('README.md', 'index.md')
+        url = f'[{title}]({full_path}{anchor})'.replace('README.md', 'index.md').replace('README.html', 'index.html')
         text = re.sub(SUMMARY_REGEX, url, text, count=1)
 
     return text