Update screenshots

[ryonakano]

Taken from the latest master (6bfce678ffd44f07daac2ae4219955c20d5214fe).

• Take 2x screenshots
• Use light style
• Fix screenshots path

[jlnr]

The embed in README.md still refers to the old filename. ~Maybe other places too~ — nope, just this one file :) PR: https://github.com/elementary/calculator/pull/235