[Translation] Fix fallback parameter name in the framework config

Author: bicpi

book/translation.rst

@@ -536,7 +536,7 @@ locale via routing.
 Fallback and Default Locale
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-If the locale hasn't been set explicitly in the session, the ``fallback_locale``
+If the locale hasn't been set explicitly in the session, the ``fallback``
 configuration parameter will be used by the ``Translator``. The parameter
 defaults to ``en`` (see `Configuration`_).