Fix a wrong schema example in docs/source/tutorial/mutations.mdx

[junseinagao]

About

I found the mistake in a page Update data with mutations in full stack tutorial.

Fixed point

Mutation's login return String but it must return User. The correct schema is defined in a page Build a schema

# YOU DON'T NEED TO COPY THIS.
type Mutation {
  bookTrips(launchIds: [ID]!): TripUpdateResponse!
  cancelTrip(launchId: ID!): TripUpdateResponse!
- login(email: String): String # login token
+ login(email: String): User # login token
}

[apollo-cla]

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