Changed level of other Deprecations subheadings to be consistent

Author: radarhere

docs/releasenotes/9.1.0.rst

@@ -45,7 +45,7 @@ Deprecations
 ============
 
 Constants
-~~~~~~~~~
+^^^^^^^^^
 
 A number of constants have been deprecated and will be removed in Pillow 10.0.0
 (2023-07-01). Instead, ``enum.IntEnum`` classes have been added.
@@ -121,7 +121,7 @@ In effect, ``viewer.show_file("test.jpg")`` will continue to work unchanged.
 ``viewer.show_file(path="test.jpg")`` instead.
 
 FitsStubImagePlugin
-~~~~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^^^^
 
 .. deprecated:: 9.1.0