minor #789 Fix yaml as @ is forbidden in YAML 1.2 (jdespatis)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #789).

Discussion
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Fix yaml as @ is forbidden in YAML 1.2

http://www.yaml.org/spec/1.2/spec.html#id2772075

(PR done on `2.5` branch, but this issue persists also in `2.6`, `2.7` and `master`)

Commits
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b6cbe24 Fix yaml as @ is forbidden in YAML 1.2

Author: fabpot

app/config/routing.yml

@@ -1,3 +1,3 @@
 app:
-    resource: @AppBundle/Controller/
+    resource: "@AppBundle/Controller/"
     type:     annotation