Jinja2 is being renamed to 'Jinja'

[geerlingguy]

In the book, there are multiple references to "Jinja2", which I should update and rename to just "Jinja".

Reference: geerlingguy/ansible-for-devops#197

[geerlingguy]

This has been fixed in the manuscript... and hopefully my brain can continue to remember to refer to just 'Jinja' and .jinja instead of 'Jinja2' and .j2!