Fix syntax

Author: ddeboer

doc/invalidation-handlers.rst

@@ -45,7 +45,7 @@ correct tags on all responses. You can add tags to the handler using::
 
 Before any content is sent out, you need to send the tag header_::
 
-    header(sprintf('%s: %s),
+    header(sprintf('%s: %s'),
         $tagHandler->getTagsHeaderName(),
         $tagHandler->getTagsHeaderValue()
     );