Fix TimerTask :execution_interval docs

[freemanoid]

This seems to be a mistake in the docs. Looking under the hood at the ScheduledTask class, you can see that Float is perfectly fine and can be passed as an execution interval. This PR changes Integer to Float in the method description to avoid any confusion.

concurrent-ruby/lib/concurrent-ruby/concurrent/scheduled_task.rb

Line 167 in e7ff945

# @param [Float] delay the number of seconds to wait for before executing the task

I've also checked that it actually works with floats:

Concurrent::TimerTask.execute({:execution_interval => 0.1}) { puts Time.now.to_f }

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[eregon]

Right and the specs already use that: https://github.com/ruby-concurrency/concurrent-ruby/blob/master/spec/concurrent/timer_task_spec.rb

[eregon]

Thanks!