Updated markdown file for existing project param

Author: Ragini-Microsoft

docs/CustomizingAzdParameters.md

@@ -21,7 +21,7 @@ By default this template will use the environment name as the prefix to prevent
 | `AZURE_ENV_IMAGETAG`       | string  | `latest`   | Set the Image tag Like (allowed values: latest, dev, hotfix)                                   |
 | `AZURE_ENV_EMBEDDING_MODEL_CAPACITY`   | integer | `80`                         | Sets the capacity for the embedding model deployment.                         |
 | `AZURE_ENV_LOG_ANALYTICS_WORKSPACE_ID` | string  | Guide to get your [Existing Workspace ID](/docs/re-use-log-analytics.md)  | Reuses an existing Log Analytics Workspace instead of creating a new one.     |
-
+| `AZURE_EXISTING_AI_PROJECT_RESOURCE_ID`    | string  | Guid to get your existing AI Foundry Project resource ID           | Reuses an existing AIFoundry and AIFoundryProject instead of creating a new one.  |
 
 
 ## How to Set a Parameter