ada: Fix section of Finalizable extension in GNAT RM

ada: Fix section of Finalizable extension in GNAT RM

The generalized finalization extension was awarded the title of curated
extension some time ago, but this wasn't reflected in the GNAT
reference manual before this patch, which moves the documentation for
generalized finalization in the curated extension section.

gcc/ada/ChangeLog:

	* doc/gnat_rm/gnat_language_extensions.rst: Fix section of Finalizable.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.