gdb/testsuite: fix comment in gdb.threads/non-ldr-exit.exp

Maybe there's something I don't understand in that test, but the comment
seems wrong.  It checks what happens when the non-leader thread does an
exit, not the leader.

gdb/testsuite/ChangeLog:

	* gdb.threads/non-ldr-exit.exp: Fix comment.

Change-Id: I35c96a70c097fa9529737874f54f3f78036008a4