docs: add the onlyBuyer modifier in function confirmPurchase()

in the function confirmPurchase(), the annotation said 'Confirm the purchase as buyer.' but it doesn't add the `onlyBuyer` modifier

docs/examples/safe-remote.rst

@@ -106,6 +106,7 @@ you can use state machine-like constructs inside a contract.
         /// is called.
         function confirmPurchase()
             external
+            onlyBuyer
             inState(State.Created)
             condition(msg.value == (2 * value))
             payable