doc(spanner): update SampleBanner name for isolation level sample (#15909)

* doc(spanner): update SampleBanner name for isolation level sample

Author: prathmeshcc

google/cloud/spanner/samples/samples.cc

@@ -5955,7 +5955,7 @@ void RunAll(bool emulator) {
   SampleBanner("spanner_directed_read");
   DirectedRead(project_id, instance_id, database_id);
 
-  SampleBanner("isolation-level-setting");
+  SampleBanner("spanner_isolation_level");
   IsolationLevelSetting(project_id, instance_id, database_id);
 
   SampleBanner("spanner_batch_client");