Fix typo in Session docs

Author: Vladimír Chalupecký

session.go

@@ -14,7 +14,7 @@ import (
 
 // Session provides access to information about an SSH session and methods
 // to read and write to the SSH channel with an embedded Channel interface from
-// cypto/ssh.
+// crypto/ssh.
 //
 // When Command() returns an empty slice, the user requested a shell. Otherwise
 // the user is performing an exec with those command arguments.