Update strip.white documentation in fread

[joshhwuu]

Closes #2492

Small update to fread documentation:

d = fread("A  ,B  \n1,2\n3,4\n", strip.white=F)
colnames(d)
# [1] "A  " "B  "
d = fread("  A,  B\n1,2\n3,4\n", strip.white=F)
colnames(d)
# [1] "  A" "  B"

This behavior was updated a while back, probably as a byproduct of another change, but the documentation was never updated

[MichaelChirico]

Thanks!