make python 3 the main python code example

Example 1: update link python is python 3

$ python --version
Python 2.7.6
$ python3 --version
Python 3.4.3
$ alias python=python3
$ python --version
Python 3.4.3

Example 2: update link python is python 3

$ unalias python
$ python --version
Python 2.7.6