Function print_stat_tree not defined for KMeans

[alesasse]

Hi Josh,
thank you for the great work!

I've spotted a micro-bug trying to run the example in the KMeans docstring. Executing the following code

x = [randn() + 5i for i in rand(Bool, 10^6), j in 1:2]
o = fit!(KMeans(2, 2), x)

returns UndefVarError: print_stat_tree not defined.

I think that the error is in stats/stats.jl, function Base.show(io::IO, o::KMeans). Comparing with line 278 in the same file, I think one simply has to add OnlineStatsBase. in line 393.

Thanks.