Use dictionary notation when referring to PressureRecord members

[rakuco]

PressureRecord is a dictionary, which is basically an ordered map.

As such, I'd expect the spec to refer to its members using the ordered map notation (foo["bar"] instead of foo.bar).

[rakuco]

While at it: a dictionary has members, not attributes, so the term should be updated in the subsections of https://wicg.github.io/compute-pressure/#the-pressurerecord-dictionary

[kenchris]

Please have a look @rakuco