Fix mean shift equation as per issue 12420 (scikit-learn#12455)

#### Reference Issues/PRs Fixes scikit-learn#12420 #### What does this implement/fix? Explain your changes. Updates the mean shift equation as per issue 12420

Author: mail-liam

doc/modules/clustering.rst

@@ -387,7 +387,7 @@ is updated according to the following equation:
 
 .. math::
 
- x_i^{t+1} = x_i^t + m(x_i^t)
+ x_i^{t+1} = m(x_i^t)
 
 Where :math:`N(x_i)` is the neighborhood of samples within a given distance
 around :math:`x_i` and :math:`m` is the *mean shift* vector that is computed for each