Lock: add a missed final for a field

Change-Id: I922d12b07aee8456af0fc490bea4b6be4d0ee4cc

src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/Lock.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/Lock.java b/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/Lock.java
index dd01966..94b908b 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/Lock.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/Lock.java
@@ -59,7 +59,7 @@
   private static final String RECLAIM_LOCK_PREFIX = "RECLAIM";
   private final DatabaseClient dbClient;
   private final String gerritInstanceId;
-  private ScheduledExecutorService heartbeatExecutor;
+  private final ScheduledExecutorService heartbeatExecutor;
   private final String projectName;
   private final String refName;