docs: remove extraneous 'params.slug' parameter on author page

[coreyshuman]

This PR updates a code snippet in the `Creating a Blog with Nuxt Content' post to match a minor change made in this PR for the source code: https://github.com/nuxtlabs/demo-blog-nuxt-content/pull/69

Per the PR description:

This PR removes the params.slug parameter from $content() calls on pages which don't have a slug param, such as the authors and tags pages. The inclusion of this undefined parameter doesn't affect the functionality of these pages since $content() ignores the undefined second parameter. Removing this unnecessary parameter just makes the code easier to understand.

[coreyshuman]

The code changes related to this documentation update have now been merged in here: https://github.com/nuxtlabs/demo-blog-nuxt-content/pull/69