minor #935 Fixed the suggested validation cache driver (peterrehm)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #935).

Discussion
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Fixed the suggested validation cache driver

In reference to symfony/symfony#16822.

Commits
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a795660 Fixed the suggested validation cache driver

app/config/config_prod.yml

@@ -3,7 +3,7 @@ imports:
 
 #framework:
 #    validation:
-#        cache: validator.mapping.cache.apc
+#        cache: validator.mapping.cache.doctrine.apc
 #    serializer:
 #        cache: serializer.mapping.cache.apc