Fix link to documentation (#620)

Author: taiki-e

core-client/src/lib.rs

@@ -3,7 +3,7 @@
 //! By default this crate does not implement any transports,
 //! use corresponding features (`tls`, `http` or `ws`) to opt-in for them.
 //!
-//! See documentation of [`jsonrpc-client-transports`](../jsonrpc_client_transports/) for more details.
+//! See documentation of [`jsonrpc-client-transports`](https://docs.rs/jsonrpc-client-transports) for more details.
 
 #![deny(missing_docs)]