Fix typos in chapter 4

[danielabrozzoni]

Also, adds a one-line explanation of what mapWallet is, for better readability

[willcl-ark]

Thanks for this. I have tried some git-fu on the second commit, just to move it to a new line (as trying to follow a single sentence per line) -- I hope it doesn't mess up attribution to you, but it looks ok locally to me 🤞🏼

[willcl-ark]

Ah it only messed up Github, phew :)

Closed in https://github.com/chaincodelabs/onboarding-to-bitcoin-core/commit/d1c28b756d138aa57a962b569cc4ed2378b3aaee