Update README.md

Under the **Supported Indicators and Functions** section, the example code for the *dict of functions by group* did not work. Fixed the example code.

README.md

@@ -454,7 +454,7 @@ for name in talib.get_functions():
     print(name)
 
 # dict of functions by group
-for group, names in talib.get_function_groups():
+for group, names in talib.get_function_groups().items():
     print(group)
     for name in names:
         print(name)