Fixed comment for pandas.unique
Timed pd.unique vs np.unique with 1_000 and 100_000 sequences lengths. In first case np.unique was faster, whereas in the second case pd.unique won.
Hence, I propose to make the doc a bit more accurate.

![image](https://user-images.githubusercontent.com/1339818/106612351-519ac100-6569-11eb-9388-1acfc39ef3a1.png)
