added writing capacilities to files and made the appropriate changes to the filedemo.c file

src/list.c

@@ -49,7 +49,7 @@ listPush(list_t* l, dataType_e type, void* element) {
             vectorPush((vector_t *)l->data, newVec);
             break;
         default:
-            perror("specify a valid vector type\n");
+            fprintf(stderr, "specify a valid vector type\n");
             exit(EXIT_FAILURE);
     }