un 'ensures' bizarrement en trop

examples/verifythis_PrefixSumRec.mlw

 (**
 
- ensures {1 The VerifyThis competition at FM2012: Problem 2}
+ {1 The VerifyThis competition at FM2012: Problem 2}
 
   see {h <a href="http://fm2012.verifythis.org/challenges">this web page</a>}