Fix spelling error; minor grammar change; describe delay when dl bins.
Fix spelling error; minor grammar change; describe delay when dl bins.

Summary:
I tried downloading a pre-built binary using `wget`; the initial
download took about 2m30s. Give the user an advisory about that wait.

Test Plan: Documentation change only; no test plan required.

Reviewed By: bobyangyf

fbshipit-source-id: 10425a1

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