MINOR: Update dev docs link on README.md (apache#38322)

### Rationale for this change

Broken link on the [arrow/README.md](https://github.com/apache/arrow/blob/main/README.md?plain=1#L102) after the landing page for the dev docs has been changed in apache#36591.
### What changes are included in this PR?

Update the link.

Authored-by: AlenkaF <frim.alenka@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>

README.md

@@ -99,4 +99,4 @@ integrations in other projects, we'd be happy to have you involved:
 [2]: https://github.com/apache/arrow/tree/main/format
 [3]: https://github.com/apache/arrow/issues
 [4]: https://github.com/apache/arrow
-[5]: https://arrow.apache.org/docs/dev/developers/contributing.html
+[5]: https://arrow.apache.org/docs/dev/developers/index.html