Minor change: making field final

Signed-off-by: Max Lambrecht <maxlambrecht@gmail.com>

java-spiffe-core/src/main/java/spiffe/workloadapi/WorkloadApiClient.java

@@ -59,7 +59,7 @@ public class WorkloadApiClient implements Closeable {
     // it is injected in each of the retryHandlers in the watch methods
     private final ScheduledExecutorService retryExecutor;
 
-    private ExecutorService executorService;
+    private final ExecutorService executorService;
 
     private boolean closed;