BUG Fix computation of RMSE

Author: luispedro

ch08/similar_movie.py

@@ -70,5 +70,5 @@ def all_estimates(reviews, k=1):
     error = (estimates - reviews)
     error **= 2
     error = error[reviews > 0]
-    rmse = np.sqrt(error).mean()
+    rmse = np.sqrt(error.mean())
     print("RMSE is {0}.".format(rmse))