fix(buffer_previewer): no preview showing when set nomodifiable (#3077)

Not showing a preview with the new changes in the latest changes of
[plenary.nvim PR
#575](https://github.com/nvim-lua/plenary.nvim/pull/575).

The error occurs when changing from `nomodifiable` to `modifiable`.
Telescope itself works, but the previews don't render.

(cherry picked from commit 1084d07cf4)

lua/telescope/previewers/buffer_previewer.lua

@@ -368,6 +368,7 @@ previewers.new_buffer_previewer = function(opts)
     else
       local bufnr = vim.api.nvim_create_buf(false, true)
       set_bufnr(self, bufnr)
+      vim.api.nvim_buf_set_option(bufnr, "modifiable", true)
 
       vim.schedule(function()
         if vim.api.nvim_buf_is_valid(bufnr) then