OvmfPkg/AmdSev/SecretDxe: fix header comment to generic naming

OvmfPkg/AmdSev/SecretDxe: fix header comment to generic naming

Commit 96201ae7bf97 ("OvmfPkg/AmdSev/SecretDxe: make secret location
naming generic", 2020-12-15) replaced references to SEV with the generic
term Confidential Computing, but missed the file header comment.  Fix
the naming in that header.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3457
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>