fix missing import in powgen reduction

Author: nvaytet

docs/instruments/external/powgen/powgen_reduction.ipynb

@@ -32,6 +32,7 @@
     "%matplotlib widget\n",
     "import scipp as sc\n",
     "import scippneutron as scn\n",
+    "import plopp as pp\n",
     "\n",
     "import ess\n",
     "from ess.diffraction.external import load_calibration\n",