Fix and test the write-did-and-query-verkey rust example code

[Wade Barnes]

There is an issue with the write-did-and-query-verkey.rs example. open_wallet returns a WalletHandle, not an i32.

https://github.com/hyperledger/indy-sdk/blob/master/docs/how-tos/write-did-and-query-verkey/rust/src/write-did-and-query-verkey.rs#L62