MdeModulePkg Variable: Fix comment typo 'end' to 'start'

MdeModulePkg Variable: Fix comment typo 'end' to 'start'

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1214
This patch only updates comment and function description, so has
no functionality impact.

This patch fixes comment typo 'end' to 'start' in GetStartPointer.

GetStartPointer for PEI and DXE has aligned function description,
but GetEndPointer does not.
This patch also aligns GetEndPointer's function description for
PEI and DXE.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>