Update README to use https://core.mojoapps.io/ urls.
Update README to use https://core.mojoapps.io/ urls.

See also #356 .

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1275333002 .

afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo