Commands given in samples' README giving ClassNotFound error

#### Steps to reproduce

  1. Follow the steps in samples' README.md

#### Code example

```java
mvn -X exec:java -Dexec.mainClass=com.example.spanner.QuickstartSample -Dexec.args="my-instance my-database"
```

#### Stack trace
```
java.lang.ClassNotFoundException: com.example.spanner.QuickstartSample
```
