example: update soroban_restore_footprint.py (#838)

With the SorobanDataBuilder set to `.set_read_only()` the restore transaction returns as `malformed`
With SorobanDataBuilder set to `set_read_write` it executes succesfully

See https://soroban.stellar.org/docs/fundamentals-and-concepts/state-expiration#restorefootprintop

examples/soroban_restore_footprint.py

@@ -33,7 +33,7 @@
         durability=stellar_xdr.ContractDataDurability.PERSISTENT,
     ),
 )
-soroban_data = SorobanDataBuilder().set_read_only([ledger_key]).build()
+soroban_data = SorobanDataBuilder().set_read_write([ledger_key]).build()
 
 tx = (
     TransactionBuilder(source, network_passphrase, base_fee=50000)