Fix name of variables for image sizes correctly

Author: keik

notebooks/chapter03_notebook/01_blocks.ipynb

@@ -204,8 +204,8 @@
    },
    "outputs": [],
    "source": [
-    "rows, cols = imdata.size\n",
-    "grid = BlockGrid(width=rows, height=cols,\n",
+    "width, height = imdata.size\n",
+    "grid = BlockGrid(width=width, height=height,\n",
     "                 block_size=4, lines_on=False)\n",
     "for block, rgb in zip(grid, imdata):\n",
     "    block.rgb = rgb\n",