docs(Introduction): Remove duplicate article
docs(Introduction): Remove duplicate article

Summary:
Hi Facebook Relay Maintainers 👋�

This pull request removes an extra “a” in the first sentence of the introduction. 🔬
Closes https://github.com/facebook/relay/pull/2252

Differential Revision: D6649569

Pulled By: kassens

fbshipit-source-id: 5ec5b264355412fc2c29272042bc098bc6aa44ce

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