[Elm] Bugfix .encode in modelTypeDiscriminator

Created by: andys8

The function is called encode. This fixed the call.

See https://github.com/OpenAPITools/openapi-generator/blob/dfa7e616c0f311f3ebf58c04287f200c8e491f08/modules/openapi-generator/src/main/resources/elm/modelTypeArray.mustache#L11

I have encountered this issue, and this is probably an easy fix. @eriktim Can you have a short look?