Use singleton scope for AccountCacheImpl class

This was a miss in the starting change

Change-Id: I4d1d2175c6b51883b8431d4b90e9577a43f5bb6e
diff --git a/src/main/java/com/googlesource/gerrit/plugins/remotegerritaccountcache/AccountCacheImpl.java b/src/main/java/com/googlesource/gerrit/plugins/remotegerritaccountcache/AccountCacheImpl.java
index ab7dcd0..0413dde 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/remotegerritaccountcache/AccountCacheImpl.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/remotegerritaccountcache/AccountCacheImpl.java
@@ -17,6 +17,7 @@
 import static com.google.gerrit.server.account.AccountCacheImpl.AccountCacheModule.ACCOUNT_CACHE_MODULE;
 import static com.google.gerrit.server.account.externalids.ExternalId.SCHEME_USERNAME;
 import static com.google.gerrit.server.git.QueueProvider.QueueType.BATCH;
+import static com.google.inject.Scopes.SINGLETON;
 
 import com.google.common.cache.CacheLoader;
 import com.google.common.cache.LoadingCache;
@@ -99,8 +100,8 @@
   public static class AccountCacheBindingModule extends AbstractModule {
     @Override
     protected void configure() {
-      bind(AccountCacheImpl.class);
-      bind(AccountCache.class).to(AccountCacheImpl.class);
+      bind(AccountCacheImpl.class).in(SINGLETON);
+      bind(AccountCache.class).to(AccountCacheImpl.class).in(SINGLETON);
     }
   }