Reword "can be compiled" to "are compiled" in vega-lite intro

[semla]

Small detail, I am wrapping my head around the vega tools and had to stop for a moment to think about "Specifications can be compiled to Vega" in the Vega-lite description on vega.github.io. First I read it as there are other options. I suggest changing to something like Specifications are compiled to Vega.