Update the font-family documentation to fix typo [tachyons-css#160]

Author: mrmrs

index.html

@@ -30,19 +30,19 @@
     <div class="db dtc v-mid w-100 tr">
 
       <a title="Documentation" href="/docs/"
-         class="f6 b dim link black-50 mr1 mr3-m mr4-l dib">
+         class="f6 fw6 dim link black-50 mr1 mr3-m mr4-l dib">
         Docs
       </a>
       <a title="Components" href="/components/"
-         class="f6 b dim link black-50 mr1 mr3-m mr4-l dib">
+         class="f6 fw6 dim link black-50 mr1 mr3-m mr4-l dib">
         Components
       </a>
       <a title="Tachyons on GitHub" href="http://github.com/mrmrs/tachyons/"
-         class="f6 b dim link black-50 mr1 mr3-m mr4-l dib">
+         class="f6 fw6 dim link black-50 mr1 mr3-m mr4-l dib">
         GitHub
       </a>
       <a title="Tachyons Npm Modules" href="/#npm"
-         class="f6 b link black-50 dim dib">
+         class="f6 fw6 link black-50 dim dib">
         Npm
       </a>
     </div>

src/templates/docs/font-family/index.html

@@ -53,7 +53,7 @@ <h3 class="f5 book sans-serif pt4 caps">Default System Sans Serif</h3>
     vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
     no sea takimata sanctus est Lorem ipsum dolor sit amet.
   </p>
-  <h3 class="f5 book sans-serif pt4 caps">Default System Sans Serif</h3>
+  <h3 class="f5 book sans-serif pt4 caps">Default System Serif</h3>
   <p class="system-serif lh-copy measure">
     Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod
     tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At