fix: Broken link

fixes #193

docs/en/core/citations.md

@@ -73,7 +73,7 @@ You can select one of the three styles in the settings → 'Citations' to mak
 
     Please note that citation *styles* can sometimes further customize how citations are actually being rendered. For instance, some citation styles in the natural sciences require citations to be referred to only by number. This specific requirement will be applied once you export a file. **Zettlr itself will always use a default in-text citation style to preview your files. Your exports therefore may differ.**
 
-For more information on how to use citations in line with Pandoc's citeproc engine, [please refer to the guide](http://pandoc.org/demo/example19/Extension-citations.html).
+For more information on how to use citations in line with Pandoc's citeproc engine, [please refer to the guide](https://pandoc.org/MANUAL.html#citations).
 
 ## Bibliography