fix: Add teardown callback for mobile guide (#183)

Users should close resources like autopass in a teardown callback. In
this case it will speed up reruns as `pass.close()` calls hyperswarm's
`.destroy()` which will clean up the DHT records, speeding up joining on
rerun.

guide/making-a-bare-mobile-app.md

@@ -237,6 +237,7 @@ fs.mkdirSync(path)
 const invite = Bare.argv[1]
 const pair = Autopass.pair(new Corestore(path), invite)
 const pass = await pair.finished()
+Bare.on('teardown', () => pass.close())
 
 await pass.ready()