Small spelling correction (#8)

templates/index.html

@@ -4,7 +4,7 @@
     {{ super() }}
 
     <p>This is a minimal web app developed with <a href="http://flask.pocoo.org/">Flask</a> framework.</p>
-    <p>The main purpose is to introduce how to implement the essentail elements in web applications with Flask, including</p>
+    <p>The main purpose is to introduce how to implement the essential elements in web applications with Flask, including</p>
     <ul>
         <li>URL Building</li>
         <li>Authentication with Sessions</li>
@@ -17,4 +17,4 @@
     </ul>
 
     <p>For more basic knowledge of Flask, you can refer to <a href="https://www.tutorialspoint.com/flask/">a tutorial on Tutorialspoint</a>.</p>
-{% endblock %}
+{% endblock %}