test(spanner): fix MutableCredentialsExample sample #20285

Job	Run time
changes	3s
bulk-filter	57s
lint	3m 27s
windows	2m 17s
gapic-libraries-bom	18m 44s
generation-config-check	1m 51s
enforcer	19m 39s
split-units (java-spanner, 8)	4m 45s
split-clirr (java-spanner)	3m 19s
split-dependencies (java-spanner)	3m 52s
split-units (java-spanner, 25)	5m 15s
split-units (java-spanner, 17)	5m 25s
split-units (java-spanner, 21)	7m 45s
split-units (java-spanner, 11)	10m 50s
units (25)	29s
units (17)	53s
units (11)	29s
units (21)	32s
units (8)	1m 17s
conditional-required-check	3s
	1h 31m 52s

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