fixed a w3 validation error

Author: Kidel

index.html

@@ -267,7 +267,7 @@ <h4><a href="https://github.com/Kidel/Netkit-Lab-Generator" target="_blank">http
                 <hr />
                 <br />
 
-                <h4 align="left">Bash file preview:</h4>
+                <h4>Bash file preview:</h4>
                 <textarea title="script" rows="15" readonly>{{ generateScript(netkit, labInfo) }}</textarea>
             </div>