MdePkg:Correct the comment for the DEBUG_NET.

MdePkg:Correct the comment for the DEBUG_NET.

Generalize the comment for the DEBUG_NET
#define DEBUG_NET  0x00004000  // SNP Driver
to Network IO Driver.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Cc: Subramanian Sriram <sriram-s@hpe.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>