fix(ui): Fix validation for select_prompt option

[mrjones2014]

Resolves: #440

How to Test

1. :lua require('legendary').find({ select_prompt = 'Custom prompt' })

Testing for Regressions

I have tested the following:

• Triggering keymaps from legendary.nvim in all modes (normal, insert, visual)
• Creating keymaps via legendary.nvim, then triggering via the keymap in all modes (normal, insert, visual)
• Triggering commands from legendary.nvim in all modes (normal, insert, visual)
• Creating commands via legendary.nvim, then running the command manually from the command line
• augroup/autocmds created through legendary.nvim work correctly