This rule is triggered when more than one space is used to separate the
heading text from the hash characters in a closed atx style heading:

```markdown
#  Heading 1  #

##  Heading 2  ##
```

To fix this, separate the heading text from the hash character by a single
space:

```markdown
# Heading 1 #

## Heading 2 ##
```

Note: this rule will fire if either side of the heading contains multiple
spaces.

Rationale: Extra space has no purpose and does not affect the rendering of
content.