docs: fix titles on docs.brew.sh

docs/New-Maintainer-Checklist.md

 # New Maintainer Checklist
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 **This is a guide used by existing maintainers to invite new maintainers. You might find it interesting but there's nothing here users should have to know.**
 
 So, there's someone who has been making consistently high-quality contributions to Homebrew for a long time and shown themselves able to make slightly more advanced contributions than just e.g. formula updates? Let's invite them to be a maintainer!

docs/Querying-Brew.md

 # Querying `brew`
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 _In this document we will be using [jq](https://stedolan.github.io/jq/) to parse JSON, available from Homebrew using `brew install jq`._
 
 ## Overview

docs/Versions.md

 # Versions
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 Now that [Homebrew/versions](https://github.com/homebrew/homebrew-versions) has been deprecated [Homebrew/core](https://github.com/homebrew/homebrew-core) supports multiple versions of formulae with a new naming format.
 
 In [Homebrew/versions](https://github.com/homebrew/homebrew-versions) the formula for GCC 6 was named `gcc6.rb` and began `class Gcc6 < Formula`. In [Homebrew/core](https://github.com/homebrew/homebrew-core) this same formula is named `gcc@6.rb` and begins `class GccAT6 < Formula`.

docs/brew-tap.md

 # Taps (third-party repositories)
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 `brew tap` adds more repos to the list of formulae that `brew` tracks, updates,
 and installs from. By default, `tap` assumes that the repos come from GitHub,
 but the command isn't limited to any one location.