Decide and stick to a writing style

[nventuro]

In https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2058, @frangio mentioned he doesn't agree with some aspects of the current writing style in our docs, including:

• having colons before a list starts
• capitalizing after colons on enumerations (like in the 'next steps' sections)
• including $ in console code snippets

We should settle on a writing style and use it throughout our docs.