Fix Plot.add transforms to accept Move, not Mark (mwaskom#3425)

Fixes the type in accordance with the intention of mwaskom#2948

Author: qiemem

seaborn/_core/plot.py

@@ -485,7 +485,7 @@ def on(self, target: Axes | SubFigure | Figure) -> Plot:
     def add(
         self,
         mark: Mark,
-        *transforms: Stat | Mark,
+        *transforms: Stat | Move,
         orient: str | None = None,
         legend: bool = True,
         data: DataSource = None,