[tex-live] minor suggested change in the TexLive 2018 documentation

[Karl Berry]

    in section 6.2 I think you should center the figure 12 and in "Command-line
    tools" change "xpdf" for  "poppler" (I think since the 2017 version uses
    poppler instead of xpdf)

Thanks for the suggestions. I'll tweak the doc again for next year :).
--best, karl.