minor symfony#10420 Update routing.rst (HobbitCodes)

This PR was merged into the 4.1 branch.

Discussion
----------

Update routing.rst

Without this the code will throw a missing prefix fatal as there is a nl prefix defined in the controller but not in the annotations yaml

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

e728cea Update routing.rst

Author: javiereguiluz

routing.rst

@@ -230,6 +230,7 @@ with a locale. This can be done by defining a different prefix for each locale
                 en: '' # don't prefix URLs for English, the default locale
                 fr: '/fr'
                 es: '/es'
+                nl: '/nl'
 
 .. _routing-requirements: