Fix class name in source file

Typo: pt0 -> pb0

Author: adorohovich

src/templates/docs/spacing/index.html

@@ -139,7 +139,7 @@ <h3 class="f5 fw4 pt4 ttu tracked">Padding Top</h3>
         <div><div class="pt6 mb2 dib bg-black white"> pt6</div></div>
         <div><div class="pt7 mb2 dib bg-black white"> pt7</div></div>
         <h3 class="f5 fw4 pt4 ttu tracked">Padding Bottom</h3>
-        <div><div class="pt0 mb2 dib bg-black white">pt0</div></div>
+        <div><div class="pb0 mb2 dib bg-black white">pb0</div></div>
         <div><div class="pb1 mb2 dib bg-black white">pb1 </div></div>
         <div><div class="pb2 mb2 dib bg-black white"> pb2</div></div>
         <div><div class="pb3 mb2 dib bg-black white"> pb3</div></div>