Reference iterative_factorial rather than recursive_factorial in spec

[davidrunger]

Also, fix the RSpec expectation for the number of #iterative_factorial method calls, since RSpec does not support specifying a certain number of method calls (in this case, a number of method calls greater than or equal to two) with negative message expectations.

Prior to this change, even after implementing #iterative_factorial correctly, the Ruby/spec/iterative_factorial_spec.rb specs would still fail with these error messages:

Failures:

  1) iterative_factorial
     Failure/Error: specify { expect(recursive_factorial(0)).to eq(1) }

     NoMethodError:
       undefined method `recursive_factorial' for #<RSpec::ExampleGroups::IterativeFactorial "example at ./spec/iterative_factorial_spec.rb:4">
     # ./spec/iterative_factorial_spec.rb:4:in `block (2 levels) in <top (required)>'

[...]

  5) iterative_factorial does not call itself
     Failure/Error:
       expect(self).to_not receive(:iterative_factorial)
         .at_least(:twice).and_call_original

     RuntimeError:
       `count` is not supported with negative message expectations
     # ./spec/iterative_factorial_spec.rb:10:in `block (2 levels) in <top (required)>'

Finished in 0.03218 seconds (files took 0.42977 seconds to load)
5 examples, 5 failures

With this change, the specs all pass when #iterative_factorial has been implemented correctly.

SPOILER: click to see an implementation of #iterative_factorial

def iterative_factorial(number)
  product = 1
  (2..number).each do |factor|
    product *= factor
  end
  product
end

[jaysonvirissimo]

LGTM 🍰

Hope all is well buddy, thanks for the contrib. 🙏🏼