Documentation out of date: headless-cms/content-model-plugins

[marklindan]

https://github.com/webiny/docs.webiny.com/blob/master/docs/how-to-guides/webiny-applications/headless-cms/content-model-plugins.mdx twice refers to

import { ContentModelPlugin } from "@webiny/api-headless-cms/content/plugins/ContentModelPlugin";
import { ContentModelGroupPlugin } from "@webiny/api-headless-cms/content/plugins/ContentModelGroupPlugin";

Which have been renamed to

import { CmsModelPlugin } from "@webiny/api-headless-cms/content/plugins/CmsModelPlugin";
import { CmsGroupPlugin } from "@webiny/api-headless-cms/content/plugins/CmsGroupPlugin";