doc: fix multiple operations example in readme

[obayemi]

In the graphql file specified in the readme for the multiple operation part, the operations are called Heights and Echo, while the struct is called UnionQuery and the text states that

Note that the struct and the operation in the GraphQL file must have the same name.

I think that's either a confusing oversight or a mistake

[obayemi]

it would probably be a good Idea to add in the begining a readme an example of a schema / query ? or just for this specific example, at least add an explanation that says that the operations in this specific file are called Heights and Echo without needing to check the linked gql file