Documentation should use est=TPOTClassifier rather than tpot=TPOTClassifier

## Context of the issue

[provide more detailed introduction to the issue itself and why it is relevant]

In the examples here: https://epistasislab.github.io/tpot/examples/

The code uses

`tpot = TPOTClassifier(generations=5, population_size=50, verbosity=2, random_state=42)`

but this should be

`est= TPOTClassifier(generations=5, population_size=50, verbosity=2, random_state=42)`

the variable `tpot` clashes with the name of the package and could cause confusing if people try to access the module later in the code.