Updated python multi-chain notebook vignette

Author: andrewherren

demo/notebooks/multi_chain.ipynb

@@ -488,7 +488,7 @@
     "bcf_model.sample(\n",
     "  X_train = X_train,\n",
     "  Z_train = Z_train,\n",
-    "  pi_train = propensity_train,\n",
+    "  propensity_train = propensity_train,\n",
     "  y_train = y_train,\n",
     "  num_gfr = num_gfr,\n",
     "  num_burnin = num_burnin,\n",
@@ -621,7 +621,7 @@
     "xbcf_model.sample(\n",
     "  X_train = X_train,\n",
     "  Z_train = Z_train,\n",
-    "  pi_train = propensity_train,\n",
+    "  propensity_train = propensity_train,\n",
     "  y_train = y_train,\n",
     "  num_gfr = num_gfr,\n",
     "  num_burnin = 0,\n",
@@ -649,7 +649,7 @@
     "bcf_model.sample(\n",
     "  X_train = X_train,\n",
     "  Z_train = Z_train,\n",
-    "  pi_train = propensity_train,\n",
+    "  propensity_train = propensity_train,\n",
     "  y_train = y_train,\n",
     "  num_gfr = 0,\n",
     "  num_burnin = num_burnin,\n",