StandaloneMmPkg: Fix few typos

Fix few typos in comments.

Cc: Achin Gupta <achin.gupta@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Signed-off-by: Antoine Coeur <coeur@gmx.fr>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>
Message-Id: <20200207010831.9046-76-philmd@redhat.com>

StandaloneMmPkg/Core/Dependency.c

@@ -372,7 +372,7 @@ MmIsSchedulable (
     // The math is done out of order, but it should not matter. That is
     // we may add in the sizeof (EFI_GUID) before we account for the OP Code.
     // This is not an issue, since we just need the correct end result. You
-    // need to be careful using Iterator in the loop as it's intermediate value
+    // need to be careful using Iterator in the loop as its intermediate value
     // may be strange.
     //
     Iterator++;