change category type to list of categories

Author: wjacobw

backend/question-service/src/dto/CreateQuestion.dto.ts

@@ -39,7 +39,7 @@ export class CreateQuestionDto {
     message:
       'Category must be Algorithm, DynamicProgramming, Array, SQL, Heap, Recursion, Graph, Sorting',
   })
-  categories: Category;
+  categories: Category[];
 
   @IsNotEmpty()
   @IsEnum(Difficulty, {