[hlx demo] references to static files are wrong

[kptdobe]

Run

hlx demo test
cd test
hlx up

style.css is not delivered.

@trieloff @tripodsan

[trieloff]

See adobe/helix-simulator#51

[tripodsan]

This is a problem of the demo code, not petridish.

[tripodsan]

(well, and petridish)

[tripodsan]

also a problem in --full