fix docstring typo: AbstractTransformation -> Transformation (#76)

Author: vpuri3

src/core.jl

@@ -119,7 +119,7 @@ end
 
 
 """
-    transform_deriv_params(trans::AbstractTransformation, x)
+    transform_deriv_params(trans::Transformation, x)
 
 A matrix describing how differentials on the parameters of `trans` flow through
 to the output of transformation `trans` given input `x`.