[ping] fix type explanation in plot.default

Dear R-devel,

I sent this in December, but I didn't get any answer...

The explanation of the 'type' parameter of plot.default is a bit confusing,
plus the stray closing parenthesis. I suggest the following small change to
match the one given in plot. Feel free to adjust at your convenience.

Apply to http://svn.r-project.org/R/trunk/src/library/graphics/man/plot.default.Rd