Fix the stupid factor 2 in the sum function.

[jonathanrocher]

The math were wrong and the unit test BLAH was failing.

(devenv)bash-3.2$ nosetests -v sprint_tutorial
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 "/Users/jrocher/.edm/envs/devenv/lib/python3.6/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/jrocher/Projects/SciPyConf/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 0.005s

FAILED (failures=1)

Fixes #24

[jonathanrocher]

@dopplershift Please take a look, it's ready!

[jonathanrocher]

Thanks for reviewing @RDastgh1 ! Closing this to keep the issue for next year's tutorial!