improve naming in ch 1 "predict_using_gain_guess" example

"weight" --> "estimate"
  "initial_guess" --> "initial_estimate"
  print_results: rename label "previous" --> "previous estimate", add comma

kf_book/gh_internal.py

@@ -191,7 +191,7 @@ def plot_gh_results(weights, estimates, predictions, time_step=0):
 
 
 def print_results(estimates, prediction, weight):
-    print('previous: {:.2f}, prediction: {:.2f} estimate {:.2f}'.format(
+    print('previous estimate: {:.2f}, prediction: {:.2f}, estimate {:.2f}'.format(
           estimates[-2], prediction, weight))