Fix 'Developing with Prisma Migrate' guide loose ends

[keerlu]

There are a few oddities in Developing with Prisma Migrate that broke the flow when I tried to follow it:

• There's an intentional typo in the schema, but it isn't fixed anywhere (this example does get used on a different page)
• There’s a section on configuring the shadow database that isn’t really used in the how-to content (also it’s not that clear on exactly which situations need the shadow database, the instructions worked fine for me without it)
• There’s a subsection on refining your schema with native type attributes that also feels oddly placed, should be integrated better or moved elsewhere as a more 'advanced' topic