README: Fix gdbus test command

It is TriggerFeedback and not Feedback (anymore). Also add a note how to
end the feedback in order to prevent loss of hearing :-).
8 jobs for master in 8 minutes and 17 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #336277
librem5
build:native-debian-bullseye

00:01:14

passed #336275
librem5
build:native-debian-buster:full

00:01:15

passed #336276
librem5
build:native-debian-buster:lib

00:01:11

 
  Test
passed #336278
librem5
test:native-debian-bullseye

00:00:53

59.7%
 
  Package
passed #336279
librem5
package:deb-debian-buster

00:01:40

passed #336280
librem5:arm64
package:deb-debian-buster:arm64

00:04:52

 
  Test Package
passed #336281
librem5
autopkgtest-debian-buster-package

00:01:15

passed #336282
librem5
lintian-debian-buster-package

00:01:08