fixed reference to old fileformats mrtrix3 package

Author: tclose

docs/source/tutorial/6-workflow.ipynb

@@ -481,7 +481,7 @@
     "from pathlib import Path\n",
     "import numpy as np\n",
     "from fileformats.medimage import Nifti1\n",
-    "import fileformats.medimage_mrtrix3 as mrtrix3\n",
+    "import fileformats.vendor.mrtrix3.medimage as mrtrix3\n",
     "from pydra.environments import docker\n",
     "from pydra.compose import workflow, python\n",
     "from pydra.tasks.mrtrix3.v3_1 import MrConvert, MrThreshold\n",