Fix section error in segmentation lecture

Error message while running "jupyter-book build ."

skimage-tutorials/lectures/4_segmentation.ipynb:20004: ERROR: Document
or section may not begin with a transition.

Author: lagru

lectures/4_segmentation.ipynb

@@ -21,7 +21,7 @@
    "source": [
     "# Segmentation\n",
     "\n",
-    "--------------\n",
+    "\n",
     "\n",
     "## Separating an image into one or more regions of interest.\n",
     "\n",