doc: fix createDiffieHellman generator type

OpenSSL does not provide a straight-forward way to implement a
non-integer generator, so createDiffieHellman never supported anything
other than a number as the generator. (This only applies to the
signature where the first argument is the size of the prime, and
therefore a number.)

Refs: https://github.com/nodejs/node-v0.x-archive/pull/7086
Refs: https://github.com/nodejs/node/pull/21782

PR-URL: https://github.com/nodejs/node/pull/31121


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>