DOC: fix cookbook groupby & transform example (#46957)

Negate first argument of `DataFrame.where`, since in pandas this must be the condition where the value should stay the same.

Author: julianste

doc/source/user_guide/cookbook.rst

@@ -511,7 +511,7 @@ Unlike agg, apply's callable is passed a sub-DataFrame which gives you access to
 
  def replace(g):
  mask = g < 0
- return g.where(mask, g[~mask].mean())
+ return g.where(~mask, g[~mask].mean())
 
  gb.transform(replace)