Fix the documentation for the PROFILE envvar passed to build scripts

[bluetech]

Closes #2084 (+ a typo fix).

[rust-highfive]

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[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit fb3213f has been approved by alexcrichton

[bors]

⌛ Testing commit fb3213f with merge 3708b12...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 3708b12 to master...