bpf/tests: Fix copy-and-paste error in double word test

This test now operates on DW as stated instead of W, which was
already covered by another test.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721104058.3755254-1-johan.almbladh@anyfinetworks.com