removed the factor 2 in the sum function

[mkb0517]

I fixed the bug in the compute sum function

[mkb0517]

test_compute.test_my_sum1 ... ok
test_compute.test_my_sum2 ... ok
test_compute.test_my_sum3 ... FAIL

======================================================================
FAIL: test_compute.test_my_sum3
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/mbrown/anaconda3/envs/devenv/lib/python3.6/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/mbrown/SciPy2018/sprint_tutorial/sprint_tutorial/tests/test_compute.py", line 17, in test_my_sum3
    assert_equal(my_sum(1, 1), 2)
AssertionError: 4 != 2

----------------------------------------------------------------------
Ran 3 tests in 0.025s

FAILED (failures=1)

Fixes #24