bug #3503 file extension correction xfliff to xliff (nixilla)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3503).

Discussion
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file extension correction xfliff to xliff

may be confusing for some readers

Commits
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583202c file extension correction xfliff to xliff

book/translation.rst

@@ -395,7 +395,7 @@ key ``Symfony2 is great``. To find the French translation, Symfony actually
 checks translation resources for several different locales:
 
 1. First, Symfony looks for the translation in a ``fr_FR`` translation resource
-   (e.g. ``messages.fr_FR.xfliff``);
+   (e.g. ``messages.fr_FR.xliff``);
 
 2. If it wasn't found, Symfony looks for the translation in a ``fr`` translation
    resource (e.g. ``messages.fr.xliff``);