Fix width calc in src

Author: johno

src/templates/docs/widths/index.html

@@ -52,7 +52,7 @@ <h3 class="f6 ttu tracked mt0">Widths</h3>
   -100 = literal value 100%
 
   -third  = calc(100% / 3)
-  -two-thirds  = calc(100% / 3)
+  -two-thirds  = calc(100% / 1.5)
   -auto  = string value auto