minor #6362 [book] Updated link to Translatable Extension (AAstakhov)

This PR was merged into the 2.3 branch.

Discussion
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[book] Updated link to Translatable Extension

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Since l3pp4rd project was moved it is better to provide link to the new project. Moreover, I think it is better to show the exact page of Translatable Extension instead of a list of extensions.

Commits
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b273c67 Updated link to Translatable Extension
b4adf83 Updated link to Translatable Extension

Author: weaverryan

book/translation.rst

@@ -717,5 +717,5 @@ steps:
 .. _`i18n`: https://en.wikipedia.org/wiki/Internationalization_and_localization
 .. _`ISO 3166-1 alpha-2`: https://en.wikipedia.org/wiki/ISO_3166-1#Current_codes
 .. _`ISO 639-1`: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
-.. _`Translatable Extension`: https://github.com/l3pp4rd/DoctrineExtensions
+.. _`Translatable Extension`: http://atlantic18.github.io/DoctrineExtensions/doc/translatable.html
 .. _`Translatable Behavior`: https://github.com/KnpLabs/DoctrineBehaviors