Update src/test/scala/p2_we_need_to_go_deeper/e4_on_a_besoin_de_la_covariance.scala

fix flatMap A=>B en A=>Sac[B]
Merge pull request #3 from mchataigner/patch-3

Update src/test/scala/p2_we_need_to_go_deeper/e4_on_a_besoin_de_la_covar...