This rule is triggered when a heading is indented by one or more spaces:

```markdown
Some text

  # Indented heading
```

To fix this, ensure that all headings start at the beginning of the line:

```markdown
Some text

# Heading
```

Note that scenarios like block quotes "indent" the start of the line, so the
following is also correct:

```markdown
> # Heading in Block Quote
```

Rationale: Headings that don't start at the beginning of the line will not be
parsed as headings, and will instead appear as regular text.