fix openapi generate example in docs (#2276) (#2278)

Author: tomkralidis

docs/source/administration.rst

@@ -32,7 +32,7 @@ To generate the OpenAPI document as JSON, run:
 
 .. code-block:: bash
 
-   pygeoapi openapi generate /path/to/my-pygeoapi-config.yml --format json --output-file /path/to/my-pygeoapi-openapi.yml
+   pygeoapi openapi generate /path/to/my-pygeoapi-config.yml --format json --output-file /path/to/my-pygeoapi-openapi.json
 
 .. note::
    Generate as YAML or JSON?  If your OpenAPI YAML definition is slow to render as JSON,