minor RBAC.sol documentation fix

[dougiebuckets]

Fixes #1055

🚀 Description

Minor RBAC.sol documentation fix where. I.e., removed "It's also recommended that you define constants in the contract, like ROLE_ADMIN below, to avoid typos." in hopes of avoid confusion.

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[frangio]

Thanks @dougiebuckets!