Fix a small typo in the doc reporter that causes mocha to crash

[jleyba]

This fixes the following scenario:

describe('Mocha', function() {
  it('should fail', function() {
    throw Error('ouch');
  });
});

$ mocha example.js --reporter doc
    <section class="suite">
      <h1>Mocha</h1>
      <dl>
        <dt class="error">should fail</dt>
        <dt class="error">should fail</dt>

Expected output:

<section class="suite">
      <h1>Mocha</h1>
      <dl>
        <dt class="error">should fail</dt>
        <dd class="error"><pre><code>throw Error('ouch');</code></pre></dd>
        <dd class="error">Error: ouch</dd>
      </dl>
    </section>

The typo was introduced in d59cc6c:
https://github.com/danielstjules/mocha/blob/d59cc6c166d0d112a62016909b33e04ed08d2274/lib/reporters/doc.js#L58

[boneskull]

@jleyba this looks good, thank you!