Rename api.HTMLMarqueeElement.bgcolor -> bgColor (#7473)
Rename api.HTMLMarqueeElement.bgcolor -> bgColor (#7473)

This corrects capitalization for the HTMLMarqueeElement API's `bgColor` feature.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data