typo fix (#493)

Author: MatthewCaseres

docs/gallery/plot_benktander.ipynb

@@ -27,7 +27,7 @@
    "metadata": {},
    "source": [
     "This example demonstrates the relationship between the Chainladder and\n",
-    "BornhuetterFerguson methods by way fo the Benktander model. Each is a\n",
+    "BornhuetterFerguson methods by way of the Benktander model. Each is a\n",
     "special case of the Benktander model where ``n_iters = 1`` for BornhuetterFerguson\n",
     "and as ``n_iters`` approaches infinity yields the chainladder.  As ``n_iters``\n",
     "increases the apriori selection becomes less relevant regardless of initial\n",