Audit docs and convert all Web3 instance variables to use `w3` name.

### What was wrong?

A while back we settled on a convention of using `w3` as the variable name for an instance of the `Web3` object.  We still have some places in the docs that use the old `web3` name.

### How can it be fixed?

Check through the docs for code examples which use the variable name `web3` to represent an instance of the `Web3` object and convert them to use `w3` instead.
