docs: add missing quotes (#2424)

Author: Akmadan23

doc/nvim-tree-lua.txt

@@ -909,19 +909,19 @@ Sign column icon precedence: git < modified < diagnostics
     Place where the git icons will be rendered.
     Can be `"after"` or `"before"` filename (after the file/folders icons)
     or `"signcolumn"` (requires |nvim-tree.view.signcolumn| enabled).
-      Type: `string`, Default: `before`
+      Type: `string`, Default: `"before"`
 
     *nvim-tree.renderer.icons.diagnostics_placement*
     Place where the diagnostics icon will be rendered.
     Can be `"after"` or `"before"` filename (after the file/folders icons)
     or `"signcolumn"` (requires |nvim-tree.view.signcolumn| enabled).
-      Type: `string`, Default: `signcolumn`
+      Type: `string`, Default: `"signcolumn"`
 
     *nvim-tree.renderer.icons.modified_placement*
     Place where the modified icon will be rendered.
     Can be `"after"` or `"before"` filename (after the file/folders icons)
     or `"signcolumn"` (requires |nvim-tree.view.signcolumn| enabled).
-      Type: `string`, Default: `after`
+      Type: `string`, Default: `"after"`
 
     *nvim-tree.renderer.icons.padding*
     Inserted between icon and filename.
@@ -1058,7 +1058,7 @@ The open command itself.
 Leave empty for OS specific default:
   UNIX:    `"xdg-open"`
   macOS:   `"open"`
-  Windows: "`cmd"`
+  Windows: `"cmd"`
 
 *nvim-tree.system_open.args*
 Optional argument list.