Fix incorrect schema example in Mutation page, add notes regarding nested input types

[seclerp]

Summary:

• Fix incorrect schema in Schema-first example of Mutations setup (this page)
• Add a note to describe a potential issue that will cause introspection schema to be invalid without proper validation and description from GraphQL/Playground side
  Closes Website: Schema-first example for Mutations page contains incorrect schema #3805

[tobias-tengler]

@seclerp Good catch. Thanks a bunch! :)

Could you maybe move the note about the nested input types to the bottom of the input-object-types.md file?

⚠ Note: Object types nested inside of an input object type need to also be declared as input types.

[seclerp]

@tobias-tengler yep, sure. Done

[tobias-tengler]

/AzurePipelines run

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