Docs: Default AMQP heartbeat delay should be 60 seconds in rabbitmq.config.example

[niclic]

The default AMQP heartbeat delay value in /docs/rabbitmq.config.example is 600 seconds. According to the offical docs, the value should be 60.

PR coming...

 %%
   %% Additional network and protocol related configuration
   %% =====================================================
   %%

   %% Set the default AMQP heartbeat delay (in seconds).
   %%
%% {heartbeat, 600},