fix(treesitter): really backup spelloptions (#22145)

see 0887ad1cbb for the original commit on
master, and 80bbba94d6 for the backport on
the branch, which forgot to backup the original setting value.

runtime/lua/vim/treesitter/highlighter.lua

@@ -88,6 +88,8 @@ function TSHighlighter.new(tree, opts)
     end
   end
 
+  self.orig_spelloptions = vim.bo[self.bufnr].spelloptions
+
   vim.bo[self.bufnr].syntax = ''
   vim.b[self.bufnr].ts_highlight = true