bugfix(sample): fix graphql sample bug (nestjs#3237)

create cat having property named id

Author: dobest27

sample/12-graphql-apollo/src/cats/cats.service.ts

@@ -6,6 +6,7 @@ export class CatsService {
   private readonly cats: Cat[] = [{ id: 1, name: 'Cat', age: 5 }];
 
   create(cat: Cat): Cat {
+    cat.id = this.cats.length + 1;
     this.cats.push(cat);
     return cat;
   }