Add author and PR link to changelog

Author: multimeric

news/changelog-1.9.md

@@ -11,7 +11,7 @@ All changes included in 1.9:
 
 - Update `esbuild` to 0.25.10
 - Update `deno` to 2.4.5
-- Update `mermaid` to 11.12.0
+- ([#13601](https://github.com/quarto-dev/quarto-cli/pull/13601)): Update `mermaid` to 11.12.0 (author: @multimeric)
 
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