Update SEND example with content-length header

[onacit]

The spec mandates that

SEND frames SHOULD include a content-length header and a content-type header if a body is present.

And the example is missing with the content-length header, with a body.

destination:/queue/a
content-type:text/plain

hello queue a
^@

[onacit]

Or you can add an errata document for an already-finalized spec.