StandaloneMmPkg: Core: Spelling error in comment

StandaloneMmPkg: Core: Spelling error in comment

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3445

This change fixed a misspelling that was not caught by spell check.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Signed-off-by: Sean Brogan <sean.brogan@microsoft.com>
Signed-off-by: Kun Qin <kuqin12@gmail.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>