Update python-built-in-functions sample code (#762)

* Fix naming in python-built-in-functions samples

- fibonacci.py: rename class Fibonaccish to Fibonacci
- shapes.py: rename Square parameter length to side

* Update README to match new tutorial title

---------

Co-authored-by: Bartosz Zaczyński <bartosz.zaczynski@gmail.com>

Author: lpozo

python-built-in-functions/README.md

@@ -1,3 +1,3 @@
-# Python's Built-in Functions: A Complete Exploration
+# Python Built-in Functions: A Complete Guide
 
-This folder provides the code examples for the Real Python tutorial [Python's Built-in Functions: A Complete Exploration](https://realpython.com/python-built-in-functions/).
+This folder provides the code examples for the Real Python tutorial [Python Built-in Functions: A Complete Guide](https://realpython.com/python-built-in-functions/).

python-built-in-functions/fibonacci.py

@@ -1,4 +1,4 @@
-class Fibonaccish:
+class Fibonacci:
     def __init__(self, initial_value=1):
         self._cache = [0, initial_value]
 

python-built-in-functions/shapes.py

@@ -11,5 +11,5 @@ def perimeter(self):
 
 
 class Square(Rectangle):
-    def __init__(self, length):
-        super().__init__(length, length)
+    def __init__(self, side):
+        super().__init__(side, side)