README.OE-Core.md: update URLs

Update URLs to what they actually redirect to.

Cc: Quentin Schulz <foss+yocto@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ec213107)
Signed-off-by: Steve Sakoman <steve@sakoman.com>