[Markdown] Spellcheck: should ignore code block with indentation

[xiaogaozi]

For example the spellcheck rule in .autocorrectrc is:

spellcheck:
  words:
    - hello = world

When the code block doesn't have indentation, the spellcheck will ignore it:

```
hello  # won't be replaced to "world"
```

But if it has indentation, the spellcheck will report error:

  ```
  hello  # will be replaced to "world"
  ```

The autocorrect version is 2.6.2