Fix typo in comments

Author: vladima

src/compiler/checker.ts

@@ -4935,11 +4935,11 @@ module ts {
             // The instanceof operator requires the left operand to be of type Any, an object type, or a type parameter type,
             // and the right operand to be of type Any or a subtype of the 'Function' interface type. 
             // The result is always of the Boolean primitive type.
-            // NOTE: do not raise error is leftType is unknown as related error was already reported
+            // NOTE: do not raise error if leftType is unknown as related error was already reported
             if (leftType !== unknownType && !isTypeAnyTypeObjectTypeOrTypeParameter(leftType)) {
                 error(node.left, Diagnostics.The_left_hand_side_of_an_instanceof_expression_must_be_of_type_any_an_object_type_or_a_type_parameter);
             }
-            // NOTE: do not raise error is right is unknown as related error was already reported
+            // NOTE: do not raise error if right is unknown as related error was already reported
             if (rightType !== unknownType && rightType !== anyType && !isTypeSubtypeOf(rightType, globalFunctionType)) {
                 error(node.right, Diagnostics.The_right_hand_side_of_an_instanceof_expression_must_be_of_type_any_or_of_a_type_assignable_to_the_Function_interface_type);
             }