docs typo, add $ to env var

Author: pdurbin

doc/sphinx-guides/source/api/native-api.rst

@@ -1179,7 +1179,7 @@ See also :ref:`batch-exports-through-the-api` and the note below:
   export PERSISTENT_IDENTIFIER=doi:10.5072/FK2/J8SJZB
   export METADATA_FORMAT=ddi
 
-  curl "$SERVER_URL/api/datasets/export?exporter=$METADATA_FORMAT&persistentId=PERSISTENT_IDENTIFIER"
+  curl "$SERVER_URL/api/datasets/export?exporter=$METADATA_FORMAT&persistentId=$PERSISTENT_IDENTIFIER"
 
 The fully expanded example above (without environment variables) looks like this: