fix iteration count

Update the epoc/iteration count noted to match the code.

samples/core/tutorials/keras/basic_text_classification.ipynb

@@ -621,7 +621,7 @@
       "source": [
         "## Train the model\n",
         "\n",
-        "Train the model for 20 epochs in mini-batches of 512 samples. This is 20 iterations over all samples in the `x_train` and `y_train` tensors. While training, monitor the model's loss and accuracy on the 10,000 samples from the validation set:"
+        "Train the model for 40 epochs in mini-batches of 512 samples. This is 40 iterations over all samples in the `x_train` and `y_train` tensors. While training, monitor the model's loss and accuracy on the 10,000 samples from the validation set:"
       ]
     },
     {
@@ -813,4 +813,4 @@
       ]
     }
   ]
-}
\ No newline at end of file
+}