fix(auto_preview): Instantly preview whenever another node is selected

simrat39/symbols-outline.nvim#119

Rather than using another config option, this should be the default,
expected behaviour for auto_preview.

Also (another difference to the PR implementation is that) there is no
need to subscribe to both CursorHold and CursorMoved.

lua/symbols-outline.lua

@@ -41,7 +41,7 @@ end
 
 local function setup_buffer_autocmd()
   if config.options.auto_preview then
-    vim.api.nvim_create_autocmd('CursorHold', {
+    vim.api.nvim_create_autocmd('CursorMoved', {
       buffer = 0,
       callback = require('symbols-outline.preview').show,
     })