Fix bug in Supervisors & Registry example

The Supervisors and Registry example is not working. I'm seeing two issues.

1. Its calling an undefined method `actor` in two places. I included a potential solution in the pull request. `celluloid/lib/celluloid/calls.rb:30:in 'check': undefined method 'actor' for <Celluloid::CellProxy(Celluloid::SupervisionGroup:0x3fc4c1844340)`

2. Second, `Celluloid::Actor[:my_actor]` on line 78 is returning nil, causing an undefined method `state`. I'm not sure why this is happening.  Inserting a `sleep` on the previous line for any amount of time seems to fix the issue. But perhaps there is a solution that address the root of the issue?

`examples/supervisors_and_registry.rb:78:in '<main>': undefined method 'state' for nil:NilClass (NoMethodError)`

line 78:
 `puts "We should now be in a clean state again: #{Celluloid::Actor[:my_actor].state}"`