doc: fix reference to API hash.final

fix a reference to a non-existent API, `hash.final()`.
It should be `hash.digest()`.

PR-URL: https://github.com/nodejs/node/pull/5050
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

doc/api/crypto.markdown

@@ -538,7 +538,7 @@ used in one of two ways:
 
 - As a [stream][] that is both readable and writable, where data is written
   to produce a computed hash digest on the readable side, or
-- Using the `hash.update()` and `hash.final()` methods to produce the
+- Using the `hash.update()` and `hash.digest()` methods to produce the
   computed hash.
 
 The `crypto.createHash()` method is used to create `Hash` instances. `Hash`