Update spec URL for nullish coalescing operator (#6449)
Update spec URL for nullish coalescing operator (#6449)

The nullish coalescing operator is no longer a proposal but instead is
now part of the ES spec.
chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data