Update 20_databases.md

[hj296]

Numbering properly. Before - In order for our custom model to be registered by Django, we'll need to do two things: --1. Register the app in our project, so that Django knows where to look for models --1. Run a command to make new migrations before running them, so that Django can create database scripts for our custom models.

After - In order for our custom model to be registered by Django, we'll need to do two things:

1. Register the app in our project, so that Django knows where to look for models
2. Run a command to make new migrations before running them, so that Django can create database scripts for our custom models.