* fix erc20 fee token decimals to 18

* lint

* Update contracts/GSN/bouncers/GSNBouncerERC20Fee.sol

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

* change location of hardcoded decimals for clarity

* remove mention of decimals from docs

* remove trailing whitespace from guide

(cherry picked from commit b8ccf8e0)