Update drvfail008 output (flag name change)

testsuite/tests/ghc-regress/deriving/should_fail/drvfail008.stderr

@@ -2,5 +2,5 @@
 drvfail008.hs:10:0:
     Can't make a derived instance of `Monad M'
       (`Monad' is not a derivable class
-       Try -fglasgow-exts for GHC's newtype-deriving extension)
+       Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension)
     In the newtype declaration for `M'