refactor(project_management): Update URL route name for ProjectMilestone model

ref: #833 #830

docs/projects/centurion_erp/user/core/markdown.md

@@ -95,7 +95,7 @@ A Model link is a reference to an item within the database. Supported model link
 | organization| `$organization-<id>` |
 | port| `$port-<id>` |
 | project| `$project-<id>` |
-| projectmilestone| `$-<id>` |
+| projectmilestone| `$project_milestone-<id>` |
 | projectstate| `$project_state-<id>` |
 | projecttask| `$-<id>` |
 | projecttype| `$-<id>` |