fixed typo 01_pytorch_workflow.ipynb

Author: jackattackyang

docs/01_pytorch_workflow.ipynb

@@ -790,7 +790,7 @@
     "\n",
     "And that's a **loss function** as well as an **optimizer**.\n",
     "\n",
-    "The rolls of these are: \n",
+    "The roles of these are: \n",
     "\n",
     "| Function | What does it do? | Where does it live in PyTorch? | Common values |\n",
     "| ----- | ----- | ----- | ----- |\n",