Escape paths containing space (because windows)

The following content was showing up in `:messages` on startup after installing.

```
[gruvbox-material] Generated C:\Users\Dakota Lorance\AppData\Local\nvim-data\lazy\gruvbox-material\after\syntax                        Failed to run `config` for gruvbox-material                                                                                            vim/_editor.lua:431: C:\Users\Dakota Lorance\.config\nvim\init.lua..nvim_exec2() called at C:\Users\Dakota Lorance\.config\nvim\init.lua:0..C:\Users\Dakota Lorance\AppData\Local\nvim-data\lazy\gruvbox-material\colors\gruvbox-material.vim[1622]..function gruvbox_material#syn_gen, line 20: Vim(set):E518: Unknown option: Lorance\AppData\Local\nvim-data\lazy\gruvbox-material\after 
```

The problem was caused by the default name of my windows user directory containing a space. 
e.g.: `C:\Users\First Last\`

Author: slyboots

autoload/gruvbox_material.vim

@@ -319,7 +319,7 @@ function! gruvbox_material#syn_gen(path, last_modified, msg) "{{{
     call gruvbox_material#ftplugin_detect(a:path)
   else
     echohl WarningMsg | echom '[gruvbox-material] Generated ' . rootpath . syntax_relative_path | echohl None
-    execute 'set runtimepath+=' . fnamemodify(rootpath, ':p') . 'after'
+    execute 'set runtimepath+=' . fnameescape(fnamemodify(rootpath, ':p')) . 'after'
   endif
 endfunction "}}}
 function! gruvbox_material#syn_write(rootpath, syn, content) "{{{