Include Multi Unlock as valid reference target

tips/TIP-0045/tip-0045.md

@@ -1116,11 +1116,6 @@ The <i>Reference Unlock</i> references a previous <i>Unlock</i> (which must not
   </tr>
 </table>
 
-##### Reference Unlock Syntactic Validation
-
-- The _Reference Unlock_ at index `i` must have `Reference < i` and the unlock at index `Reference` must be a _Signature
-  Unlock_.
-
 Example: Consider a <i>Transaction</i> containing the <i>UTXO Inputs</i> 0, 1 and 2, where 0 and 2 are both spending
 outputs belonging to the same Ed25519 address `A` and 1 is spending from a different address `B`. This results in the
 following structure of the <i>Unlocks</i> part:
@@ -1131,6 +1126,11 @@ following structure of the <i>Unlocks</i> part:
 | 1     | A _Signature Unlock_ holding the Ed25519 signature for address `B`.                  |
 | 2     | A _Reference Unlock_ which references 0, as both require the same signature for `A`. |
 
+##### Reference Unlock Syntactic Validation
+
+- The _Reference Unlock_ at index `i` must have `Reference < i` and the unlock at index `Reference` must be a _Signature
+  Unlock_ or _Multi Unlock_.
+
 ##### Reference Unlock Semantic Validation
 
 - It must hold for the `Unlock` at index `Reference`, to which the _Reference Unlock_ at index `i` points and the