Typo Fixed

Index.html had a typo.

index.html

@@ -161,7 +161,7 @@ layout: default
                         </div>
                         <div class="back">
                             <div>
-                                <p>Like defining your own commands ? Printrun lets you easily define macros (with arbitraty python, shell and G-Code combinations) and add custom buttons for them within Pronterface.</p>
+                                <p>Like defining your own commands ? Printrun lets you easily define macros (with arbitrary python, shell and G-Code combinations) and add custom buttons for them within Pronterface.</p>
                             </div>
                             <h4>Macros &amp; custom buttons</h4>
                         </div>