doc: fix crypto "decipher.setAAD()" typo

PR-URL: https://github.com/nodejs/node/pull/9782
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

doc/api/crypto.md

@@ -311,7 +311,7 @@ added: v1.0.0
 -->
 
 When using an authenticated encryption mode (only `GCM` is currently
-supported), the `cipher.setAAD()` method sets the value used for the
+supported), the `decipher.setAAD()` method sets the value used for the
 _additional authenticated data_ (AAD) input parameter.
 
 ### decipher.setAuthTag(buffer)