Fix formula in spherical model docs

[mmaelicke]

closes #164

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (d96b063) 90.74% compared to head (88f06af) 90.74%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   90.74%   90.74%           
=======================================
  Files          23       23           
  Lines        2475     2475           
=======================================
  Hits         2246     2246           
  Misses        229      229           

Files	Coverage Δ
skgstat/models.py	100.00% <ø> (ø)

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