Update "arc call-conduit" instructions in Conduit API console for required "--"

Summary: See PHI1912. Ref T13491. "arc" now requires "--" when stdin is not a TTY; provide this argument for users.

Test Plan: Viewed example in console, saw "--". Executed example.

Maniphest Tasks: T13491

Differential Revision: https://secure.phabricator.com/D21482