Correct comment about Verified Boot key on devices with custom root of trust.

Updating the comment to match the test expectation. The attested Verified
Boot key should contain the user-set root of trust if the Verified Boot
state is "SelfSigned" (i.e. "yellow").

Bug: 376832222
Test: n/a (comment update)
Change-Id: Ie3c43157f05e40fa42b107768af561644ff5d30f