Support emoji in README Markdown

[dbeatty10]

As reported by @eddowh:

This page on the dbt Package Hub (hub.getdbt.com) doesn't seem to support emojis. should be ✅ instead of :white_check_mark::

Additional information

We had similar Markdown issues reported and/or fixed here:

• dbt Hub README doesn't match the GitHub README format #1583
• Fix the display of summary blocks #1780
• Put Github badges on one row #1786