testsuite: fix two newly-running -Wstringop-overflow test directives

This didn't show up until the previous commit which fixed the directive
syntax. The indexing was off for the notes.

gcc/testsuite/ChangeLog:

	* gcc.dg/Wstringop-overflow-79.c: Fix index for notes.
	* gcc.dg/Wstringop-overflow-80.c: Ditto.