BUG: Use the literal superclass name in itkTypeMacro

Use the literal superclass name instead of the `Superclass` alias in
`itkTypeMacro`: the string is used in an ITK macro which is using the
input for run-time type information (although currently not using the
superclass input):
https://github.com/InsightSoftwareConsortium/ITK/blob/8844ee6549ebdc1294a17d471f8a65311440bd74/Modules/Core/Common/include/itkMacro.h#L438

include/itkMGHImageIO.h

@@ -50,7 +50,7 @@ class MGHIO_EXPORT MGHImageIO : public ImageIOBase
   itkNewMacro(Self);
 
   /** Run-time type information (and related methods). */
-  itkTypeMacro(MGHImageIO, Superclass);
+  itkTypeMacro(MGHImageIO, ImageIOBase);
 
   /*-------- This part of the interfaces deals with reading data. ----- */