Add missing double-quote in doc (#498)

docs/documentation/overview/modular.html

@@ -47,7 +47,7 @@ doc-subtab: modular
         What if you only want the <strong>button</strong> styles instead?
       </p>
 {% highlight sass %}
-@import "bulma/sass/utilities/_all
+@import "bulma/sass/utilities/_all"
 @import "bulma/sass/elements/button.sass"
 {% endhighlight %}
       <p>