Update download.html

Small edit

Author: nds9

download.html

@@ -7,7 +7,7 @@
     <section id="one">
         <div class="container">
             <header class="major">
-                <h2>Downloading</h2>
+                <h2>Download</h2>
             </header>
             <p>OpenSBLI is an open-source code, released under the GNU General Public License. The source code can be obtained from the project's GitHub repository: <a href="http://github.com/opensbli/opensbli">http://github.com/opensbli/opensbli</a></p>
         </div>