DOC: Fix text formatting of imshow_extent example (matplotlib#30471)

* DOC: Make text font normal rather than italics

* DOC: Update the title of imshow_extent.py

Author: star1327p

galleries/users_explain/artists/imshow_extent.py

@@ -3,8 +3,8 @@
 
 .. _imshow_extent:
 
-*origin* and *extent* in `~.Axes.imshow`
-========================================
+Positioning and orientation of `~.Axes.imshow` images
+=====================================================
 
 :meth:`~.Axes.imshow` allows you to render an image (either a 2D array which
 will be color-mapped (based on *norm* and *cmap*) or a 3D RGB(A) array which