Update example code to better fit pep8 code style (tensorflow#3303)

Adding spaces around a variable assignment to be a better example to people following the tutorial.

Author: zac-hopkinson

docs/get_started.ipynb

@@ -229,7 +229,7 @@
         "              loss='sparse_categorical_crossentropy',\n",
         "              metrics=['accuracy'])\n",
         "\n",
-        "log_dir=\"logs/fit/\" + datetime.datetime.now().strftime(\"%Y%m%d-%H%M%S\")\n",
+        "log_dir = \"logs/fit/\" + datetime.datetime.now().strftime(\"%Y%m%d-%H%M%S\")\n",
         "tensorboard_callback = tf.keras.callbacks.TensorBoard(log_dir=log_dir, histogram_freq=1)\n",
         "\n",
         "model.fit(x=x_train, \n",