Improve error message in config.py (#532)

Use the correct argument to the error message in the function defined at line 73 of the config.py file.

cosmos/config.py

@@ -81,7 +81,7 @@ def validate_project(self) -> None:
         }
         for name, path in mandatory_paths.items():
             if path is None or not Path(path).exists():
-                raise CosmosValueError(f"Could not find {name} at {project_yml_path}")
+                raise CosmosValueError(f"Could not find {name} at {path}")
 
     def is_manifest_available(self) -> bool:
         """