chore: fix tag for isolation level sample

[skuruppu]

Update the sample tag to be consistent with other samples.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/spanner/SpannerSample.java:1558, tag spanner_isolation_level

You are about to delete 1 region tag.

• samples/snippets/src/main/java/com/example/spanner/SpannerSample.java:1558, tag spanner_isolation_level_settings

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