One last correction of non-pythonic strings.

MutualRecursion.py

@@ -93,10 +93,10 @@ def divide(left_expression, right_expression):
 
 
 operator_map = {
-    "+": add,
-    "-": subtract,
-    "*": multiply,
-    "/": divide
+    '+': add,
+    '-': subtract,
+    '*': multiply,
+    '/': divide
 }