add using statement in doc

Author: selfmadecode

doc/Rsa.md

@@ -17,6 +17,7 @@ It includes methods for generating RSA key pairs, encrypting data with a public
 
 ```csharp
 using SafeCrypt.Helpers;
+using SafeCrypt.RsaEncryption;
 
 var rsaKeyPair = KeyGenerators.GenerateRsaKeys(2048);