Improve bash completion for `service create|update --detach`

bash completion usually completes boolean options that default to `true`
with the `=false` suffix because that is the only syntax that really
changes behavior.
For this option, I implemented completion falsely.

Signed-off-by: Harald Albers <github@albersweb.de>

contrib/completion/bash/docker

@@ -3081,7 +3081,7 @@ _docker_service_update_and_create() {
 	"
 
 	local boolean_options="
-		--detach -d
+		--detach=false -d=false
 		--help
 		--no-healthcheck
 		--read-only