Add one more note to the 7.2.3 changelog before publishing (dart-lang/markdown#628)

pkgs/markdown/CHANGELOG.md

diff --git a/pkgs/markdown/CHANGELOG.md b/pkgs/markdown/CHANGELOG.md
index 658d8b6..04d3d9a 100644
--- a/pkgs/markdown/CHANGELOG.md
+++ b/pkgs/markdown/CHANGELOG.md
@@ -3,6 +3,7 @@
 * Fix an issue with checkbox list items separated with blank lines (#602).
 * Require package `web: '>=0.4.2 <2.0.0'`.
 * Fix several `RangeError` hazards in links (#623).
+* Export `LinkReferenceDefinitionSyntax` publicly (#626).
 
 ## 7.2.2