Fix comment in ada-typeprint.c

A comment in ada-typeprint.c mentions the Unchecked_Variant pragma.
However, this does not exist, and the comment should actually mention
Unchecked_Union.

gdb/ChangeLog
2020-03-11  Tom Tromey  <tromey@adacore.com>

	* ada-typeprint.c (print_choices): Fix comment.