DOC: fix Ward's distance formula

Just a minor sign error in the description.

From `_hierarchy_distance_update.pxi`
``` cython
cdef double _ward(double d_xi, double d_yi, double d_xy,
                  int size_x, int size_y, int size_i):
    cdef double t = 1.0 / (size_x + size_y + size_i)
    return sqrt((size_i + size_x) * t * d_xi * d_xi +
                (size_i + size_y) * t * d_yi * d_yi -
                size_i * t * d_xy * d_xy)
```