analysis id example: sort values to ensure order is deterministic (`git diff` in CI pipeline)

examples/freesurfer_hipposf.ipynb

@@ -197,7 +197,7 @@
     {
      "data": {
       "text/plain": [
-       "[None, 'T2']"
+       "array(['T2', None], dtype=object)"
       ]
      },
      "execution_count": 5,
@@ -206,7 +206,7 @@
     }
    ],
    "source": [
-    "list(volume_frame['analysis_id'].unique())"
+    "volume_frame['analysis_id'].sort_values().unique()"
    ]
   },
   {