This rule is triggered when spaces are missing inside 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 is missing spaces.

Rationale: Violations of this rule can lead to improperly rendered content.