Update concepts.html

Fixes a typo in the docs

docs/documentation/customize/concepts.html

@@ -32,7 +32,7 @@
 {% include docs/elements/anchor.html name="Global Sass Variables" %}
 
 {% capture markdown %}
-Bulma uses Sass variables globally defines in 2 files located in the `utilities` folder:
+Bulma uses Sass variables globally defined in 2 files located in the `utilities` folder:
 
 * `initial-variables.scss` where you define variables by literal value
   * **colors** like `$blue: hsl(229, 53%, 53%)`
@@ -186,4 +186,4 @@
   --bulma-title-color: #fff;
   --bulma-title-line-height: 1.6;
 }
-{% endhighlight %}
+{% endhighlight %}