fix typo in notebook 17 (#335)

17_foundations.ipynb

@@ -1093,7 +1093,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Another useful wat of simplifying tensor manipulations is the use of Einstein summations convention."
+    "Another useful way of simplifying tensor manipulations is the use of Einstein summations convention."
    ]
   },
   {