Added another missing import inside transformer module.

IPython/nbconvert/transformers/__init__.py

@@ -2,6 +2,7 b''
 from .activatable import ActivatableTransformer
 from .base import ConfigurableTransformer
 from .extractfigure import ExtractFigureTransformer
+from .revealhelp import RevealHelpTransformer
 from .latex import LatexTransformer
 from .sphinx import SphinxTransformer