Fix incorrect reference to _font-weight.css

Typo: _font-weights.css -> _font-weight.css

Author: johanbrook

docs/typography/font-weight/index.html

@@ -178,7 +178,7 @@ <h4 class="f4 mv0 fw6 dib mr4">tachyons-font-weight</h4>
             <dd class="db pl0 ml0 f4 f2-ns b">1</dd>
           </dl>
         </div>
-        <kbd>src/_font-weights.css</kbd>
+        <kbd>src/_font-weight.css</kbd>
       </header>
 <pre class="ph3 ph5-ns">
 <code class="code" style="font-size: .75rem;">