docs(samples): add tag to statement timeout sample

[olavloite]

Add a tag to the StatementTimeoutExample, so we can include this sample in the documentation.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/spanner/StatementTimeoutExample.java:35, tag spanner_set_statement_timeout

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