Fix webpack docs

docs/documentation/customize/with-webpack.html

@@ -140,7 +140,7 @@ module.exports = {
   </div>
 {% endcapture %}
 
-{% capture step_3.5 %}
+{% capture step_3_5 %}
   <div class="content">
     <p>
       Create a <code>webpack.config.js</code> file:
@@ -233,10 +233,10 @@ Wrote CSS to /path/to/mybulma/css/mystyles.css
 %}
 
 <hr>
-    
+
 {% include components/step.html
   title="3.5. Create a webpack config (Webpack 4)"
-  content=step_3.5
+  content=step_3_5
 %}
 
 <hr>