Fix indentation in prepare_a_slice_via_images.ipynb

Indentation was incorrect. Added one more space.

Author: ywsong2

fabric_examples/mflib/prepare_a_slice_via_images.ipynb

@@ -71,7 +71,7 @@
     "from fabrictestbed_extensions.fablib.fablib import FablibManager as fablib_manager\n",
     "\n",
     "try:\n",
-    "   fablib = fablib_manager()\n",
+    "    fablib = fablib_manager()\n",
     "                    \n",
     "    fablib.show_config()\n",
     "except Exception as e:\n",