upgrade addon docs to 0.6.12

Author: ryanto

package.json

@@ -35,7 +35,7 @@
     "broccoli-plugin": "^1.3.0",
     "broccoli-stew": "^2.0.0",
     "ember-cli": "~3.1.4",
-    "ember-cli-addon-docs": "0.6.11",
+    "ember-cli-addon-docs": "0.6.12",
     "ember-cli-addon-docs-yuidoc": "^0.2.1",
     "ember-cli-dependency-checker": "^2.0.0",
     "ember-cli-deploy": "^1.0.2",