Advanced Indexing - Update Exercise solution

The small exercise at the bottom of the page declared two DataArrays, but only used one. The solution was still correct but confusing, so I reduced the needed variable declaration to the required one and renamed.

Author: jomey

intermediate/indexing/advanced-indexing.ipynb

@@ -374,10 +374,9 @@
     "\n",
     "indices = np.array([0, 2, 4])\n",
     "\n",
-    "xs_da = xr.DataArray(indices, dims=\"points\")\n",
-    "ys_da = xr.DataArray(indices, dims=\"points\")\n",
+    "index_da = xr.DataArray(indices, dims=\"points\")\n",
     "\n",
-    "subset_da = da.sel(x=xs_da, y=xs_da)\n",
+    "subset_da = da.sel(x=index_da, y=index_da)\n",
     "subset_da\n",
     "```\n",
     ":::\n",