Update 00-planning_and_overall_intro.md

[jikunbupt]

URL of get-pip.py has changed.

$# curl -O https://bootstrap.pypa.io/2.7/get-pip.py
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 936 100 936 0 0 545 0 0:00:01 0:00:01 --:--:-- 545
$# python get-pip.py

Hi there!

The URL you are using to fetch this script has changed, and this one will no
longer work. Please use get-pip.py from the following URL instead:

https://bootstrap.pypa.io/pip/2.7/get-pip.py