BUGFIX: 3-arg subtraction was not implemented.

The following change allows use of sub like  ..

Now added! This is a bug, not an enhacenement.

src/calc.py

@@ -9,12 +9,12 @@ def add(a, b, third_operand = 0):
     """
     return a + b + third_operand
 
-def sub(a, b):
+def sub(a, b, c=0):
     """
     Subtract some numbers
 
     ```py
     sub(8, 3) # 5
     ```
     """
-    return a - b
+    return a - b - c