Fixed email-ext plugin code

Change-Id: Ic18c23dee2a3758d7d7026b07cd22ab9ecfd4d82
diff --git a/Dockerfile b/Dockerfile
index e0f6692..c09c453 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -84,7 +84,7 @@
         description-setter \
         discard-old-build \
         docker-workflow \
-        email-ext-plugin \
+        email-ext \
         extended-choice-parameter \
         extensible-choice-parameter \
         gerrit-trigger \