docs: update entity-manager-api.md (#5340)
docs: update entity-manager-api.md (#5340)

typo:
await manager.count(User, { firstName: "Timber" }, "age", 3); -> await manager.decrement(User, { firstName: "Timber" }, "age", 3);
gintsgints/typeorm,typeorm/typeorm,typeorm/typeorm,typeorm/typeorm,gintsgints/typeorm