Code blocks should not be spaced 4 times, as this will
prevent the code from being colored. They should
also be spaced the same as the lists they are a part
of, to make reading easier.

- Also makes other minor Markdown fixes that were near the main fixes.

So we compare `expect_any_instance_of` with
`expect_next_instance_of`, but still mention
`allow_any_instance_of`.

I've followed the [upgrade guide](https://github.com/thoughtbot/factory_bot/blob/4-9-0-stable/UPGRADE_FROM_FACTORY_GIRL.md) and ran these two commands:

```
grep -e FactoryGirl **/*.rake **/*.rb -s -l | xargs sed -i "" "s|FactoryGirl|FactoryBot|"
grep -e factory_girl **/*.rake **/*.rb -s -l | xargs sed -i "" "s|factory_girl|factory_bot|"
```

Signed-off-by: Rémy Coutable <remy@rymai.me>

This is now covered by rubocop-rspec.

[ci skip]