releasing: s/master/main/

This default branch name has been changed. Update RELEASING
accordingly.

Signed-off-by: Simon Ser <contact@emersion.fr>
6 jobs for releasing-main-branch in 1 minute and 22 seconds (queued for 1 second)
Status Name Job ID Coverage
  Containers
passed debian #15587643

00:00:13

 
  Build
passed meson-aarch64 #15587646

00:01:07

passed meson-armhf #15587647

00:01:07

passed meson-i386 #15587645

00:01:05

passed meson-ppc64el #15587648

00:00:11

passed meson-x86_64 #15587644

00:01:04