Update folly::experimental::coro docs to account for getCPUExecutor deprecation
Update folly::experimental::coro docs to account for getCPUExecutor deprecation

Summary: title

Reviewed By: yfeldblum

Differential Revision: D28345979

fbshipit-source-id: ea55db34e19d3881c65559172d4fc9df4808fdad

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly