Typo in documentation in "gqlgen/docs/content/reference/changesets.md"

### What happened?
Typo in gqlgen/docs/content/reference/changesets.md
```graphql
type Mutation {
updateUser(id: ID!, changes: UserChanges!): User
}

type UserChanges {
name: String
email: String
}
```

### What did you expect?
```graphql
type Mutation {
updateUser(id: ID!, changes: UserChanges!): User
}

input UserChanges { // change type to input
name: String
email: String
}
```

### Minimal graphql.schema and models to reproduce
- Irrelevant

### versions
- [v0.16.0](https://gqlgen.com/master/v0.16.0).