fix lint issue

Author: ldetmer

samples/snippets/src/main/java/com/example/spanner/MutableCredentialsExample.java

@@ -19,10 +19,10 @@
 // [START spanner_mutable_credentials]
 
 import com.google.auth.oauth2.ServiceAccountCredentials;
+import com.google.cloud.spanner.MutableCredentials;
 import com.google.cloud.spanner.Spanner;
 import com.google.cloud.spanner.SpannerOptions;
 import com.google.cloud.spanner.admin.database.v1.DatabaseAdminClient;
-import com.google.cloud.spanner.MutableCredentials;
 import java.io.FileInputStream;
 import java.io.IOException;
 import java.nio.file.Files;