Adding "type" to the examples of Screenshot object

[webmaxru]

I think that adding type to the Example 1 and Example 2 of the Screenshot object will improve developer experience.

This member comes from ImageResource object and it's optional (so actually the examples are syntactically correct). But all other optional members are included there and it might cause some confusion (or at least some extra time for the research). To improve DX, I suggest adding "type": "image/png", to both examples.