Fix use of rlang::is_string() inside of toggle_switch()

[cpsievert]

Introduced by #709. Fixes the following error (discovered through and will fix https://rstudio.github.io/shinycoreci/results/2023/09/11/#315-bslib-input-switch)

library(shiny)
library(bslib)

ui <- page_fixed(
  input_switch("x", "Switch")
)

server <- function(input, output, session) {
  toggle_switch("x")
}

shinyApp(ui, server)

Error in rlang::is_string(id, n = 1) : unused argument (n = 1)

[gadenbuie]

Oops 🙊 thank you!