doc: update link of ICU data slicer

PR-URL: nodejs/node#43483
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

tools/icu/README.md

@@ -36,4 +36,4 @@ Note:
 * [The ICU Homepage][ICU]
 
 [ICU]: http://icu-project.org
-[ICU data slicer]: https://github.com/unicode-org/icu/blob/master/docs/userguide/icu_data/buildtool.md
+[ICU data slicer]: https://github.com/unicode-org/icu/blob/HEAD/docs/userguide/icu_data/buildtool.md