update python package name

At least on ubuntu 20.04, the python3 package is now 3.8, but the package 'python3' points to the current best version of python available without needing to specialize a minor version and should require fewer changes to the document.

docs/installation/3-netbox.md

@@ -12,7 +12,7 @@ Begin by installing all system packages required by NetBox and its dependencies.
 ### Ubuntu
 
 ```no-highlight
-sudo apt install -y python3.6 python3-pip python3-venv python3-dev build-essential libxml2-dev libxslt1-dev libffi-dev libpq-dev libssl-dev zlib1g-dev
+sudo apt install -y python3 python3-pip python3-venv python3-dev build-essential libxml2-dev libxslt1-dev libffi-dev libpq-dev libssl-dev zlib1g-dev
 ```
 
 ### CentOS