minor symfony#6921 Fix var_dumper advanced usage link (ogizanagi)

This PR was merged into the 2.7 branch.

Discussion
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Fix var_dumper advanced usage link

Fixes the "advanced usages" link from the http://symfony.com/doc/current/components/var_dumper.html#the-dump-function section's note.

I've performed a quick search for similar errors in other documents, but was unable to find any other. 😃

Commits
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86f1348 Fix var_dumper advanced usage link

Author: wouterj

components/var_dumper.rst

@@ -64,7 +64,7 @@ current PHP SAPI:
 .. note::
 
     If you want to catch the dump output as a string, please read the
-    `advanced documentation <advanced>`_ which contains examples of it.
+    :doc:`advanced documentation </components/var_dumper/advanced>` which contains examples of it.
     You'll also learn how to change the format or redirect the output to
     wherever you want.