bash completion for `stack deploy --prune`

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

contrib/completion/bash/docker

@@ -4017,7 +4017,7 @@ _docker_stack_deploy() {
 
 	case "$cur" in
 		-*)
-			local options="--compose-file -c --help --with-registry-auth"
+			local options="--compose-file -c --help --prune --with-registry-auth"
 			__docker_daemon_is_experimental && options+=" --bundle-file"
 			COMPREPLY=( $( compgen -W "$options" -- "$cur" ) )
 			;;