doc: fix return type

IIUC, crypto.createDiffieHellmanGroup(name) and its original crypto.getDiffieHellman(groupName) will return a instance of DiffieHellmanGroup which is not a sub type of DiffieHellman.

const instnace = crypto.createDiffieHellmanGroup("modp14")

console.log(instnace instanceof crypto.DiffieHellmanGroup)
// true
console.log(instnace instanceof crypto.DiffieHellman)
// false

on Node.js v12.10.0.

Checklist

• documentation is changed or added
• commit message follows commit guidelines