update code

Author: RezaSaadatyar

Code/oop.ipynb

@@ -34,6 +34,7 @@
  "$\\;\\;\\;\\;\\;$ Using the `__init__ `method<br/>\n",
  "$\\;\\;\\;\\;\\;$ Initializing instance attributes<br/>\n",
  "$\\;\\;\\;\\;\\;$ Setting default and custom parameters<br/>\n",
+ "▪ [Link 1](https://www.w3schools.com/python/python_classes.asp)$\\;\\;\\;\\;\\;$ ▪ [Link 2](https://realpython.com/python3-object-oriented-programming/)$\\;\\;\\;\\;\\;$ ▪ [Link 3](https://pynative.com/python-classes-and-objects/)$\\;\\;\\;\\;\\;$\n",
  "\n",
  "**2. Inheritance**<br/>\n",
  "▪ Single Inheritance<br/>\n",