Finish adding beginner math challenges

Author: bgroveben

challenges/4.1.Addition/lesson_tests.py

@@ -8,5 +8,5 @@ def test_main(self):
         self.assertEqual(total, 20)
 
 # To run the tests from the console:
-# Make sure that you are in the 'addition' directory
+# Make sure that you are in the '4.1.Addition' directory
 # ⇒  python3 -m unittest lesson_tests

challenges/4.2.Subtraction/lesson.md

@@ -6,10 +6,11 @@ In Python, an integer (int) is one of 3 distinct numeric types.
 In this exercise, you will subtract two integers using the minus (-) operator.
 - https://docs.python.org/3.6/library/operator.html#operator.sub
 - https://docs.python.org/3/library/operator.html#mapping-operators-to-functions
+
 ```
 >>> 4 - 2
 2
 ```
 
 **_Instructions:_**  
-**Change the 0 so that total will equal 10.**  
+**Change the 0 so that total will equal 10.**