Update doc for Session::subsystem

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Author: NobodyXu

src/session.rs

@@ -182,9 +182,11 @@ impl Session {
     ///
     /// ## Sftp subsystem
     ///
-    /// To use sftp subsystem, it is recommended to enable feature `openssh-sftp-client`,
+    /// To use sftp subsystem, it is recommended to use crate [`openssh-sftp-client`],
     /// then use the following code to construct a sftp instance:
     ///
+    /// [`openssh-sftp-client`]: https://crates.io/crates/openssh-sftp-client
+    ///
     /// ```rust,no_run
     /// # use std::error::Error;
     /// # #[cfg(feature = "native-mux")]