Add "# Good" for unnamed arguments on long lines

[johann-wagner]

Most other code chunks in the section include a comment indicating which part is # Good and # Bad. Currently, the current code chunk is as follows:

map(x, f,
    extra_argument_a = 10,
    extra_argument_b = c(1, 43, 390, 210209)
)

Whilst the code chunk only includes one section of code, I think for consistency and clarity, it would assist in having the # Good comment clearly indicate that this is an example of good coding syntax.

# Good
map(x, f,
    extra_argument_a = 10,
    extra_argument_b = c(1, 43, 390, 210209)
)

[hadley]

This example is going to go away as part of fixing #189.