Update ChangeLog.md

Author: kleisauke

ChangeLog.md

@@ -21,7 +21,7 @@ See docs/process.md for more on how version tagging works.
 3.1.73 (in development)
 -----------------------
 - libunwind was updated to LLVM 19.1.4. (#22394)
-- mimalloc was updated to 2.1.7. (#21548)
+- mimalloc was updated to microsoft/mimalloc@9b75377. (#21548)
 
 3.1.72 - 11/19/24
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