minor #15071 [Translation] Fix link to Translatable Extension documentation (sebpacz)

This PR was merged into the 4.4 branch.

Discussion
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[Translation] Fix link to Translatable Extension documentation

The site http://atlantic18.github.io/DoctrineExtensions/ returns 404.

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Commits
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a4063c4 [Translation] Fix link to Translatable Extension documentation

Author: javiereguiluz

translation.rst

@@ -610,5 +610,5 @@ Learn more
 .. _`ICU MessageFormat`: https://unicode-org.github.io/icu/userguide/format_parse/messages/
 .. _`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`: http://atlantic18.github.io/DoctrineExtensions/doc/translatable.html
+.. _`Translatable Extension`: https://github.com/doctrine-extensions/DoctrineExtensions/blob/main/doc/translatable.md
 .. _`Translatable Behavior`: https://github.com/KnpLabs/DoctrineBehaviors