Use `--example STRING` rather than `:linenumber` to future-proof about hw-ruby-intro HOT 2 CLOSED

rspec spec/part1_spec.rb --example "#sum"
Run options: include {:full_description=>/\#sum/}

Ruby intro part 1
  #sum
    should be defined
    returns correct sum [20 points] (FAILED - 1)
    works on the empty array [10 points] (FAILED - 2)
  #sum_to_n
    should be defined
    returns true when any two elements sum to the second argument [30 points] (FAILED - 3)
    returns false for any single element array [5 points] (FAILED - 4)
    returns false for an empty array [5 points] (FAILED - 5)

rspec spec/part1_spec.rb --example '#sum '