docs: Remove outdated getting started step

We no longer use Postgres.

For:
#16

doc/tutorial.md

@@ -6,5 +6,4 @@
 2. Install [Pipenv](https://pipenv.pypa.io/en/latest/index.html): `pip install --user pipenv`.
 3. Activate a virtual environment: `pipenv shell`.
 4. Install dependencies: `pipenv sync --dev`.
-5. [Set up Postgres locally](./howto.md#set-up-postgres-locally)
-6. Run the development server: `invoke dev` or `inv dev`.
+5. Run the development server: `invoke dev` or `inv dev`.