doc: fix "sign.verify" typo in crypto doc.

Fix typo in example

PR-URL: https://github.com/nodejs/node/pull/7411
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/crypto.md

@@ -809,7 +809,7 @@ verify.end();
 
 const public_key = getPublicKeySomehow();
 const signature = getSignatureToVerify();
-console.log(sign.verify(public_key, signature));
+console.log(verify.verify(public_key, signature));
   // Prints true or false
 ```