referring to LaTeX package pgfplots in the manual

[tpapp]

The manual says we use PGFPlots for the LaTeX package, while I recently (erroneously) suggested that we use pgfplots.

I would like to make a PR that makes ensures consistency, but it would be good to decide what to use first.

pgfplots is the actual LaTeX package name. LaTeX docs usually use small caps for packages, which Markdown does not really have (unless we want to muck with HTML, which I don't think we want).

PGFPlots is what we more or less use now, it stands out in text better.

I suggest we use italicized pgfplots.