docs: Update SpannerSample.java to align with best practices

[adhiggs]

Thanks for stopping by to let us know something could be better!

PLEASE READ: If you have a support contract with Google, please create an issue in the support console instead of filing on GitHub. This will ensure a timely response.

Please run down the following list and make sure you've tried the usual "quick fixes":

• Search the issues already opened: https://github.com/googleapis/java-spanner/issues
• Check for answers on StackOverflow: http://stackoverflow.com/questions/tagged/google-cloud-platform

If you are still having issues, please include as much information as possible:

static void readOnlyTransaction in the sample uses a try-with-resources statement in the second statement, but not the first statement. The associated pull request adds a try-with-resources statement to the first statement to make it compliant with best practices.

For more information, see b/372711568.