minor symfony#11303 added requirements for {_format} (stoccc)

This PR was merged into the 3.4 branch.

Discussion
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added requirements for {_format}

I added requirements for the `{_format}` parameter: in this example `_format` should be `xml` or `html`

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Commits
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f2ebfed added requirements for {_format}

templating/formats.rst

@@ -51,7 +51,7 @@ be configured so that ``/about-us`` sets the request format to ``html`` while
 special ``_format`` placeholder in your route definition::
 
     /**
-     * @Route("/{slug}.{_format}", defaults={"_format"="html"})
+     * @Route("/{slug}.{_format}", defaults={"_format"="html"}, requirements={"_format"="html|xml"}))
      */
     public function showAction(Request $request, $slug)
     {