Update username-password guide

[porcellus]

With the latest version the guide has some issues, that causes it to return WRONG_CREDENTIALS_ERROR when using a username. I suspect this is because of this: https://github.com/supertokens/supertokens-node/blob/7e33a06f4283a150600a5eabda31615fc5827023/lib/ts/recipe/emailpassword/api/implementation.ts#L766

We need to update the guide to take that into account (and/or investigate if we can do something else to accommodate this usecase)