Fix tab input value updating for BS4 dropdowns

[cpsievert]

This fixes an oversight made in #3372, when we switched (back to) emit-ing li a.dropdown-item instead of a.dropdown-item. To verify the fix, notice how (before this change) selecting tab B results in an input value of NULL (when it should be "B"):

library(shiny)

ui <- fluidPage(
  theme = bslib::bs_theme(version = 4),
  tabsetPanel(
    id = "tabs",
    tabPanel("A", "a"),
    navbarMenu("menu", tabPanel("B", "b"))
  )
)

server <- function(input, output, session) {
  observe({
    print(input$tabs)
  })
}

shinyApp(ui, server)