link to 18F GH repo in layout

Just fixes a single link

other spots to update in layout:

```html
    <div class="cfa-logos">
      
      <a href="http://codeforamerica.org/"><img src="/assets/cfa_logo_footer.png" class="right" alt="Code for America" /></a>
      <span class="plus">&#43;</span>
      <a href="http://openoakland.org/"><img src="/assets/openoaklandlogo.png" class="right" alt="City of Oakland" /></a>
      <span class="plus">&#43;</span>
      <a href="http://www2.oaklandnet.com"><img src="/assets/OaklandCityTree.png" class="right" alt="City of Oakland" /></a>
    </div>
```