fix mask spec in atomic-fetch-bit-op docs
fix mask spec in atomic-fetch-bit-op docs

Summary: The bit param is 0-based. So, for bit 1, the mask is `0b10`.

Reviewed By: luciang

Differential Revision: D32160394

fbshipit-source-id: cfef250b3035d8c7e29b2f36848482888801781c

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