Update default value for origin

The default value for `origin` is `upper` as described in [matplotlib official documentation](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.imshow.html)

src/seaborn_image/_context.py

@@ -99,7 +99,7 @@ def reset_defaults():
  mpl.rcParams.update(mpl.rcParamsDefault)
 
 
-def set_image(cmap="deep", origin="lower", interpolation="nearest", despine=False):
+def set_image(cmap="deep", origin="upper", interpolation="nearest", despine=False):
  """
  Set deaults for plotting images
 
@@ -108,7 +108,7 @@ def set_image(cmap="deep", origin="lower", interpolation="nearest", despine=Fals
  cmap : str, optional
  Colormap to use accross images, by default to "deep".
  origin : str, optional
- Image origin - same as in `matplotlib.pyplot.imshow`, by default "lower".
+ Image origin - same as in `matplotlib.pyplot.imshow`, by default "upper".
  interpolation : str, optional
  Image interpolation - same as in `matplotlib.pyplot.imshow`, by default "nearest".
  despine : bool, optional