plot_id should be ugd_id in documentation

[barryrowlingson]

The example in Roxygen here uses plot_id()

httpgd/R/httpgd.R

Line 215 in 087ebf5

#' hgd_url(plot_id(), width = 800, height = 600)

but the correct function appears to be ugd_id() from the unigd package, as mentioned in the Roxygen arguments:

endpoint: API endpoint. The default, ‘"live"’ is the HTML/JS plot
          viewer. Can be set to a numeric plot index or plot ID (see
          ‘unigd::ugd_id()’) to obtain the direct URL to the SVG.