Fix dead link in README.md (#934)

README.md

@@ -95,7 +95,7 @@ The usage documentation for this library lives in a handful of places:
 
 You can also refer to:
 
- * the [documentation](https://developers.stellar.org/api/introduction/) for the Horizon REST API (if using the `Horizon` module) and
+ * the [documentation](https://developers.stellar.org/network/horizon) for the Horizon REST API (if using the `Horizon` module) and
  * the [documentation](https://soroban.stellar.org/docs/reference/rpc) for Soroban RPC's API (if using the `SorobanRpc` module)
 
 ### Usage with React-Native