Fix TimerTask docs to not refer to #execute as "blocking"

[bensheldon]

Also fixes some small other oddities I found when running the code example.

Closes #794

[ioquatix]

Can you double check this removal is correct?

When I ran the code, I got:

Boom! 
Boom! Boom! 
Boom! Boom! Boom! 
Boom! Boom! Boom! Boom! 
Boom! Boom! Boom! Boom! Boom! 
Stopping...

[bensheldon]

Oops, sorry about that. I just re-ran it myself and also got 5 booms. I'll make a correction 🤦🏻

❯ bundle exec irb
Ignoring concurrent-ruby-ext-1.2.2 because its extensions are not built. Try: gem pristine concurrent-ruby-ext --version 1.2.2
irb(main):001:0> require 'concurrent/timer_task'
=> true
irb(main):002:1*     timer_task = Concurrent::TimerTask.new(execution_interval: 1) do |task|
irb(main):003:1*       task.execution_interval.to_i.times{ print 'Boom! ' }
irb(main):004:1*       print "\n"
irb(main):005:1*       task.execution_interval += 1
irb(main):006:2*       if task.execution_interval > 5
irb(main):007:2*         puts 'Stopping...'
irb(main):008:2*         task.shutdown
irb(main):009:1*       end
irb(main):010:0>     end
=>
#<Concurrent::TimerTask:0x0000000105b49d48
...
irb(main):011:0>
irb(main):012:0>     timer_task.execute
=>
#<Concurrent::TimerTask:0x0000000105b49d48
 @StopEvent=
  #<Concurrent::Event:0x0000000105b49ca8
   @__Condition__=#<Thread::ConditionVariable:0x00000001058d1160>,
   @__Lock__=#<Thread::Mutex:0x00000001058d1188>,
   @iteration=0,
   @set=false>,
 @StoppedEvent=
  #<Concurrent::Event:0x0000000105b49c58
   @__Condition__=#<Thread::ConditionVariable:0x00000001058d10c0>,
   @__Lock__=#<Thread::Mutex:0x00000001058d10e8>,
   @iteration=0,
   @set=false>,
 @__Condition__=#<Thread::ConditionVariable:0x00000001058d2178>,
 @__Lock__=#<Thread::Mutex:0x00000001058d21a0>,
 @auto_terminate=true,
 @copy_on_deref=nil,
 @do_nothing_on_deref=true,
 @dup_on_deref=nil,
 @execution_interval=1.0,
 @executor=
  #<Concurrent::SafeTaskExecutor:0x0000000105b49cf8
   @__Condition__=#<Thread::ConditionVariable:0x00000001058d1958>,
   @__Lock__=#<Thread::Mutex:0x00000001058d1980>,
   @exception_class=StandardError,
   @task=#<Proc:0x00000001058d1ef8 (irb):2>>,
 @freeze_on_deref=nil,
 @observers=
  #<Concurrent::Collection::CopyOnNotifyObserverSet:0x00000001058d14d0
   @__Condition__=#<Thread::ConditionVariable:0x00000001058d1340>,
   @__Lock__=#<Thread::Mutex:0x00000001058d1368>,
   @observers={}>,
 @run_now=nil,
 @running=#<Concurrent::AtomicBoolean:0x00000001058d1638 value:true>,
 @value=nil>
irb(main):013:0> Boom!
Boom! Boom!
Boom! Boom! Boom!
Boom! Boom! Boom! Boom!
Boom! Boom! Boom! Boom! Boom!
Stopping...
irb(main):013:0>

[ioquatix]

Thanks!