tiny typo

[shenhai-ran]

1. chapter 7, page 18, the line below the first graph, numpy.array(), missing a y :)
2. code block 21 is not in Bk1_Ch07_17.ipynb, and stop_time = time.time() is missing for the second time calculation.

[Visualize-ML]

谢谢你，确实有问题。请继续批评指正。