SecurityPkg TPM2: Update desc for param Buffer of GetDigestListSize()

SecurityPkg TPM2: Update desc for param Buffer of GetDigestListSize()

To make the description more clear, update the description
for parameter Buffer of GetDigestListSize() to
"Buffer to hold copied TPML_DIGEST_VALUES compact binary.".

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by : Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>