Summarize function throws error for docs with less than 5 sentences.

[Ayushk4]

Here's how to replicate the error

julia> using TextAnalysis
	
julia> s = StringDocument("Assume this short Document. We will use this to show working of summarize function. This document has too foo sentences.")
julia> TextAnalysis.summarize(s)
ERROR: BoundsError: attempt to access 3-element Array{Int64,1} at index [1:5]

[Ayushk4]

This is an easy to solve bug, changing the following to check for min b/w ns and the number of sentences after sentence_tokenizing should fix this.

TextAnalysis.jl/src/summarizer.jl

Line 10 in 3b046d2

return sentences[sort(sortperm(vec(p), rev=true)[1:ns])]