Fix Error in BsBv formula

According to lines:
https://github.com/InsightSoftwareConsortium/ITKBoneMorphometry/blob/b9a1292e1b3413f25fef01a07b7e16661d61ca3d/include/itkBoneMorphometryFeaturesFilter.h#L159
https://github.com/InsightSoftwareConsortium/ITKBoneMorphometry/blob/b9a1292e1b3413f25fef01a07b7e16661d61ca3d/include/itkBoneMorphometryFeaturesFilter.h#L120
https://github.com/InsightSoftwareConsortium/ITKBoneMorphometry/blob/b9a1292e1b3413f25fef01a07b7e16661d61ca3d/include/itkBoneMorphometryFeaturesFilter.h#L107

From the code the formula stated in the paper does not agree.  The formula in the paper doesn't agree with the literature as well.

Thanks for contributing this code!

doc/itkBoneMorphometry.tex

@@ -148,7 +148,7 @@ \section{Bone Morphometry Features Available}
 
 \textbf{Bone surface density} or BsBv (which stands for  stands for Bone Surface Bs over Bone Volume Bv) gives an indication on how many bone lining cells cover a given volume of bone (Bv).
 \begin{equation} \label{eqn:BsBv}
-BsBv = 2 \frac{TbN}{BsBv}
+BsBv = 2 \frac{TbN}{BvTv}
 \end{equation}
 
 \textbf{Trabecular thickness} (TbTh) is determined by filling maximal spheres into the structure using a distance transform. Then the average thickness of all maximal spheres is calculated to give an estimate of mean TbTh.