Improved TestHelper module docstring

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

htest/Test/Ganeti/TestHelper.hs

 {-# LANGUAGE TemplateHaskell #-}
 
-{-| Unittest helpers for Haskell components
+{-| Unittest helpers for TemplateHaskell components.
 
 -}