add description to swiss roll exercise

Author: Kucharssim

exercises/flow-matching-datasaurus.ipynb

@@ -206,7 +206,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "One we defined our model classes, we can instantiate a new flow matching model object."
+    "One we defined our model class, we can instantiate a new flow matching model object."
    ]
   },
   {