update readme and tests to not make doWhileLoop condition redundant

[jd2rogers2]

@sgharms
goes with #59

tests could've been passed before with the following (meaning without the incrementVariable() from the readme, with this being irrelevant i think some students were getting confused)

function doWhileLoop(arr){
  do {
    arr.pop();
  } while (arr.length);
  return arr;
}