minor symfony#16907 [Validator] Updating ICU documentation link. (cyrilverloop)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] Updating ICU documentation link.

The ICU documentation has moved to : https://unicode-org.github.io/icu/userguide/locale/

Commits
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8c48908 Updating ICU documentation link.

Author: javiereguiluz

reference/constraints/Locale.rst

@@ -117,6 +117,6 @@ Parameter        Description
 
 .. include:: /reference/constraints/_payload-option.rst.inc
 
-.. _`ICU format locale IDs`: http://userguide.icu-project.org/locale
+.. _`ICU format locale IDs`: https://unicode-org.github.io/icu/userguide/locale/
 .. _`ISO 639-1`: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
 .. _`ISO 3166-1 alpha-2`: https://en.wikipedia.org/wiki/ISO_3166-1#Current_codes