minor #3414 Update format_date.rst (ThomasLandauer)

This PR was submitted for the 3.x branch but it was merged into the 2.x branch instead.

Discussion
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Update format_date.rst

Adding link to `format_datetime` - I hope the link syntax is correct :-)

Commits
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ce747dc Update format_date.rst

doc/filters/format_date.rst

@@ -5,7 +5,7 @@
     The ``format_date`` filter was added in Twig 2.12.
 
 The ``format_date`` filter formats a date. It behaves in the exact same way as
-the ``format_datetime`` filter, but without the time.
+the :doc:`format_datetime<format_datetime>` filter, but without the time.
 
 .. note::