removed key prop from tutorial examples
removed key prop from tutorial examples

Summary:
Link to open-source doc page: https://fblitho.com/docs/tutorial

Key props are not necessary since Litho autogenerates them. So they are removed from tutorial examples to prevent confusing people that they are mandatory.

Reviewed By: mihaelao

Differential Revision: D26633519

fbshipit-source-id: 703aa6138fbbb5ce5278990ca20b99322d1d5ab1

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