fix: broken link in libs/langchain README (#10920)

**Description**
Fixes broken link to `CONTRIBUTING.md` in `libs/langchain/README.md`.

Because`libs/langchain/README.md` was copied from the top level README,
and because the README contains a link to `.github/CONTRIBUTING.md`, the
copied README's link relative path must be updated. This commit fixes
that link.

libs/langchain/README.md

@@ -92,4 +92,4 @@ For more information on these concepts, please see our [full documentation](http
 
 As an open-source project in a rapidly developing field, we are extremely open to contributions, whether it be in the form of a new feature, improved infrastructure, or better documentation.
 
-For detailed information on how to contribute, see [here](.github/CONTRIBUTING.md).
+For detailed information on how to contribute, see [here](../../.github/CONTRIBUTING.md).