Replace PackageInfo#signingCertificateHistory field doc reference.

During API review, PackageInfo#signingCertificateHistory was changed
to PackageInfo#signingInfo.  This was not reflected everywhere in
the documentation.  Update it.

Bug: 74831530
Test: Doc change only, builds.
Change-Id: I497cdd732dffacf376ac05f7ccd181004c947210