example/init: run rivertile in background

There have been multiple "bug reports" from users that have added more
configuration using riverctl after this line starting rivertile and were
confused why that configuration has no effect.

It's not our job to explain how POSIX shell works, but we can make the
example a bit more foolproof for those who do not yet have experience
with it

 example/init | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/example/init b/example/init
index 269a38e..b95a180 100755
--- a/example/init
+++ b/example/init
@@ -157,4 +157,4 @@ riverctl csd-filter-add app-id "gedit"
 # Set the default layout generator to be rivertile and start it.
 # River will send the process group of the init executable SIGTERM on exit.
 riverctl default-layout rivertile
-rivertile -view-padding 6 -outer-padding 6
+rivertile -view-padding 6 -outer-padding 6 &