Correct `ERC20` to `ERC-20`

[jlwllmr]

Describe the bug

The former occurs quite often throughout the docs — per the Consensys Editorial Style Guide, it should beERC-20.

Steps to reproduce

n/a

Expected behavior

n/a

Link(s)

n/a

[mzkrasner]

@jlwllmr my contribution: #943