Add Initializer as a term to 2.1

[logananglin98]

Is your feature request related to a problem? Please describe.
Constructor's are referred to as initializers in Python. 2.1 only has constructor as a term.

Describe the solution you'd like
Initializer should be added as a term alongside constructor in this section. This should be done in the same way that both "instance variable" and "attributes" are treated as synonymous terms in this section.

[pearcej]

This chapter needs a redo, so please do not spend time on this. I will do it.