I'm reading Qiskit's documentation. https://qiskit.org/textbook/ch-gates/multiple-qubits-entangled-states.html#3.2-Entangled-States- and they show qubits entangled as $|00\rangle$ (both qubits spin down) or $|11\rangle$ (both qubits spin up). Also written as 

$$
\frac{1}{\sqrt{2}}(|00\rangle+|11\rangle)
$$
 
From my understanding, entanglement requires one particle to be spin up and the other spin down. How is the above entangled state possible?