nixpkgs/neovim: fix blink keymap fallbacks

This was falling back to mappings only but we want <C-D> to map back to
the default behavior.

.config/nixpkgs/neovim/plugins.lua

@@ -406,8 +406,8 @@ require('blink.cmp').setup({
   },
   keymap = {
     ['<Enter>'] = { 'select_and_accept', 'fallback' },
-    ['<C-u>'] = { 'scroll_documentation_up', 'fallback_to_mappings' },
-    ['<C-d>'] = { 'scroll_documentation_down', 'fallback_to_mappings' },
+    ['<C-u>'] = { 'scroll_documentation_up', 'fallback' },
+    ['<C-d>'] = { 'scroll_documentation_down', 'fallback' },
   },
 })