Update Paganin doc

nabu/preproc/phase.py

@@ -144,14 +144,7 @@ class PaganinPhaseRetrieval(object):
 
           F = 1 + \\frac{\\delta}{\\beta} \\lambda D \\pi |k|^2
 
-        where k is the wave vector, computed as
-
-        .. math::
-
-          k_l = \\frac{1}{P} (\\frac{-1}{2} + \\frac{l}{N-1})
-
-        where P is the pixel size, N the number of pixels in one direction,
-        and :math:`l \\in [0, N-1]`.
+        where k is the wave vector.
 
 
         References