Fix typo in config.yml

Introduced by #855, but its hard to test in PR considering how config.yml is written

Author: malfet

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: cimg/ruby2.6-node
+      - image: cimg/ruby:2.6-node
     steps:
       - run:
           name: Checkout