Updated @shopify/shopify-app-session-storage-prisma's README with latest Session model schema

[mmestiyak]

WHY are these changes introduced?

Fixes #0000

The Session model schema in the documentation was outdated and did not reflect the latest structure in the schema.prisma file. This update ensures developers have accurate information when implementing session storage with Prisma.

WHAT is this pull request doing?

This PR updates the Session model schema in the README to match the latest version. The updated schema includes all required fields and optional fields for accurate representation.

Updated schema:

model Session {
  id            String    @id
  shop          String
  state         String
  isOnline      Boolean   @default(false)
  scope         String?
  expires       DateTime?
  accessToken   String
  userId        BigInt?
  firstName     String?
  lastName      String?
  email         String?
  accountOwner  Boolean?
  locale        String?
  collaborator  Boolean?
  emailVerified Boolean?
}


## Type of change

- [ ] Patch: Bug (non-breaking change which fixes an issue)

[mmestiyak]

I have signed the CLA!

[Arkham]

Thanks for your contribution!