readme: correct what it claims

The example output predated the humidity column and the cross target listed
three platforms where the Makefile builds six. It also documented seven of
fifteen flags, and said three HTTP calls per run when a custom air: column
makes four.

The forecast sample is now real output rather than a hand-written one, so it
cannot drift from the program again without someone noticing, and the flag
list is complete with a pointer to prognosis(1) for the detail.

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