MdePkg: Update comment for PcdDefaultTerminalType

MdePkg: Update comment for PcdDefaultTerminalType

Update comment for PcdDefaultTerminalType, as a new terminal type
TTYTERM has been added with type value of 4.

The new type is NOT defined in UEFI SPEC v2.5.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18023 6f19259b-4bc3-4df7-8a09-765794883524