Replace `:ack => true` with `:manual_ack => true` in Ruby tutorial
Replace `:ack => true` with `:manual_ack => true` in Ruby tutorial

Bunny has deprecated `:ack` in favor of `:manual_ack` (https://github.com/ruby-amqp/bunny/commit/9bfd6fc33bf150cc74423e13628576cb2e102317). More importantly, we don't want the tutorial to suggest that there are different semantics for the two, so we should use one consistently.

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