Fixup xarray dataarray filter

When I combined these in to one statement, I didn't notice or test that
it worked. `ndvi` is not defined at the stage the where clause is
evaluated.

Author: mfisher87

modules/06-geojupyter/demo.ipynb

@@ -224,7 +224,7 @@
     "    \"time\",\n",
     "    keep_attrs=True,\n",
     ").where(\n",
-    "    ndvi < 1\n",
+    "    lambda ndvi: ndvi < 1\n",
     ").compute()\n",
     "\n",
     "ndvi.plot.imshow()"