fix(twist): add missing denominator in Twist3.pitch calculation

The pitch calculation was missing the denominator, returning
an incorrect value. Pitch should be normalized by |w|²:

pitch = (w · v) / (w · w)

This represents the translation distance per radian along the screw axis.
All 25 twist tests pass.

Author: petercorke

spatialmath/twist.py

@@ -945,7 +945,7 @@ def pitch(self):
             >>> S.pitch
 
         """
-        return np.dot(self.w, self.v)
+        return np.dot(self.w, self.v) / np.dot(self.w, self.w)
 
     def line(self):
         """