Correct links to README files ot index.html

The summary uses the right path for those files, but not the mardown
files link like [bla](./bla/README.md).
The cause of the problem was that IndexPreprocessor renamed README.md
files to index.md without replacing in the links.

TODO: use a case-insensitive replace instead.

src/preprocess/index.rs

@@ -39,6 +39,7 @@ impl Preprocessor for IndexPreprocessor {
                         path.set_file_name("index.md");
                     }
                 }
+                ch.content = ch.content.replace("README.md", "index.md");
             }
         });