MdePkg/Library/Dxe: Fix various typos

MdePkg/Library/Dxe: Fix various typos

Fix various typos in documentation, comments and strings,
and remove the duplicated 'the'.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Antoine Coeur <coeur@gmx.fr>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>
Message-Id: <20200207010831.9046-25-philmd@redhat.com>