clarify parameter definitions for roads() state only length 1 county can have length > 1

[tphilippi]

Could you perhaps reword the documentation for the roads() function to clarify that only a single value is allowed for the state parameter, but a vector of values is supported for the county parameter?

Based on the documentation calling both the state and county parameters vectors, I attempted to request 3 counties in 2 states:
myroads <- tigris::roads(state = c("28", "28", "22"), county = c("149", "049", "065"))
That throws an error because the grepl condition has length > 1. Inspection of the code shows that roads() only handles a single value for state.

My use case shouldn't be unique: I have an AoA I need to download roads for; I use:

counties <- tigris::counties(cb = TRUE)
needCty <- sf::st_intersection(counties, AoA)

The obvious next line would be:

rd <- tigris::roads(needCty$STATEFP, needCty$COUNTYFP)

which fails when my AoA intersects 2 states.

I don't think it's worth changing roads() to support multiple states; that can be handled via dplyr as long as users know they have to call 1 state at a time (& I can give you a short example of that code).

My requested edit is in bold:

#' @param state A character vector of length 1 of the two-digit FIPS code of the state of the county
#' you'd like to download the roads for. Can also be state name or abbreviation
#' (case-insensitive).
#' @param county A character vector of the three-digit FIPS code of the county or counties you'd like
#' the roads for. Can also be a vector of county names.