docs: Update SpannerSample.java to align with best practices #10668

Job	Run time
units (11)	2m 57s
units (17)	2m 57s
units (21)	2m 33s
units (8)	3m 21s
units-with-multiplexed-session (11)	3m 24s
units-with-multiplexed-session (17)	3m 30s
units-with-multiplexed-session (8)	3m 27s
units-with-multiplexed-session (21)	3m 9s
windows	7m 39s
clirr	1m 19s
dependencies (17)	1m 26s
javadoc	2m 33s
lint	1m 13s
	39m 28s

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