Set flux_executor_nesting in notebook (#526)

* Set flux_executor_nesting in notebook

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

notebooks/3-hpc-allocation.ipynb

@@ -322,7 +322,9 @@
     }
    ],
    "source": [
-    "with Executor(backend=\"flux_allocation\", flux_executor_pmi_mode=\"pmix\") as exe:\n",
+    "with Executor(\n",
+    "    backend=\"flux_allocation\", flux_executor_pmi_mode=\"pmix\", flux_executor_nesting=True\n",
+    ") as exe:\n",
     "    fs = exe.submit(calc_nested)\n",
     "    print(fs.result())"
    ]