docs: Update SpannerSample.java to align with best practices #3623

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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
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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.