Add changelog for 6.4.12 (#6452)

* add changelog for 6.4.12

* fix changelog marker

Author: blink1073

CHANGELOG.md

@@ -14,6 +14,14 @@ Use `pip install pip --upgrade` to upgrade pip. Check pip version with
 
 <!-- <START NEW CHANGELOG ENTRY> -->
 
+## 6.4.12
+
+([Full Changelog](https://github.com/jupyter/notebook/compare/v6.4.11...6.4.12)
+
+- Address security advisory [GHSA-v7vq-3x77-87vg](https://github.com/jupyter/notebook/security/advisories/GHSA-v7vq-3x77-87vg)
+
+<!-- <END NEW CHANGELOG ENTRY> -->
+
 ## 6.4.11
 
 ([Full Changelog](https://github.com/jupyter/notebook/compare/6.4.10...3911672959fcd35cf4a1b1ad7c9c8a5651c17ae6))
@@ -35,8 +43,6 @@ Use `pip install pip --upgrade` to upgrade pip. Check pip version with
 
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-<!-- <END NEW CHANGELOG ENTRY> -->
-
 ## 6.4.9
 
 ([Full Changelog](https://github.com/jupyter/notebook/compare/v6.4.7...9e3a7001117e64a24ead07b888bd055fdd66faf3))