wrong slugify result for some heading situation

[magicdawn]

e.g http://magicdawn.ml/clean-code-javascript/#/

there are some md code like

## **Variables**
### Use meaningful and pronounceable variable names

GitHub renders like

<h2>
  <a id="user-content-variables" class="anchor" href="#variables" aria-hidden="true">
    <svg>...</svg>
  </a>
  <strong>Variables</strong
</h2>

And docsify renders

<h2 id="strongvariablesstrong">
  <a href="#/#strongvariablesstrong" data-id="strongvariablesstrong" class="anchor">
    <span>
      <strong>Variables</strong>
    </span>
  </a>
</h2>

cause

results

Manually anchor broken