fix wording

Author: vladak

docs/index.html

@@ -178,7 +178,7 @@ <h2><a name="final_assignment">Final Lab Assignment</a></h2>
 </tbody></table>
 
 <p>You can hand over the assignment anytime before the deadline.
-This can be handy to esp. if you need to finish your semester earlier
+This can be useful esp. if you need to finish your semester earlier
 (that may apply to Erasmus students).
 
 <a name="consultations"><h2>Consulations</h2></a>