Fix mermaid rendering in milestone dashboard (#18214)

Fixes: #18200

Author: silverwind

templates/user/dashboard/milestones.tmpl

@@ -117,7 +117,7 @@
 								</div>
 							{{end}}
 							{{if .Content}}
-								<div class="content">
+								<div class="markup content">
 									{{.RenderedContent|Str2html}}
 								</div>
 							{{end}}