fix: Incomplete variable

[devinoue]

Problem

In examples/markdown/markdown.mjs, the Italic rule body doesn't work.

Example

Adding __Something__, meaning italic, to the markdown in examples/markdown/test/data/test.md will result in the following error

error: Uncaught ReferenceError: content is not defined
                        return ['italic', a.children.map((c) => content()).join('')];

Solution

Add c.