Switch row, columns in active_countour example

The dimension order of row (r) and column (c) seem to be switched. This
aligns the tutorial with the gallery example at [1].

[1] https://scikit-image.org/docs/stable/auto_examples/edges/plot_active_contours.html

Author: lagru

lectures/4_segmentation.ipynb

@@ -259,13 +259,13 @@
     "    \"\"\"Generate points defining a circle on an image.\"\"\"\n",
     "    radians = np.linspace(0, 2*np.pi, resolution)\n",
     "\n",
-    "    c = center[1] + radius*np.cos(radians)\n",
     "    r = center[0] + radius*np.sin(radians)\n",
+    "    c = center[1] + radius*np.cos(radians)\n",
     "    \n",
-    "    return np.array([c, r]).T\n",
+    "    return np.array([r, c]).T\n",
     "\n",
     "# Exclude last point because a closed path should not have duplicate points\n",
-    "points = circle_points(200, [100, 220], 100)[:-1]"
+    "points = circle_points(resolution=200, center=[100, 220], radius=100)[:-1]"
    ]
   },
   {
@@ -284,8 +284,8 @@
    "outputs": [],
    "source": [
     "fig, ax = image_show(astronaut)\n",
-    "ax.plot(points[:, 0], points[:, 1], '--r', lw=3)\n",
-    "ax.plot(snake[:, 0], snake[:, 1], '-b', lw=3);"
+    "ax.plot(points[:, 1], points[:, 0], '--r', lw=3)\n",
+    "ax.plot(snake[:, 1], snake[:, 0], '-b', lw=3);"
    ]
   },
   {