Replace hard-coded iris column vectors with names() calls

[Copilot]

Long hard-coded column name vectors like c('Sepal.Length', 'Sepal.Width', 'Petal.Length', 'Petal.Width') are verbose and brittle. Replace with names(iris)[-5] in the parallel coordinate examples.

Changes

• R/coordinate.R — roxygen example for coord_() parallel coordinate
• examples/coordinates.Rmd — live parallel coordinate code chunk

# Before
g2(iris, position = c('Sepal.Length', 'Sepal.Width',
    'Petal.Length', 'Petal.Width'), color = 'Species', ...)

# After
g2(iris, position = names(iris)[-5], color = 'Species', ...)