Capitalises the first character and lowercases the rest of each element.
Examples
x<-generate_string(nchar=10,vector=LETTERS,vector_length=10)
proper(x)
#> [1] "Cyqvzoymzk" "Bjykjgmvwo" "Zepgwayvby" "Cwnlpxtibw" "Lqjrdkxwcn" "Uxuimtvzbv" "Tpupoicfsg" "Ladhwikjki" "Agjbxfnkqk" "Bvynuqjbuc"