docs: correct spelling (#1024)

stirng > string

Author: duncan

packages/docs/docs/config/README.md

@@ -102,7 +102,7 @@ A function to control what files should have `<link rel="preload">` resource hin
 - Type: `boolean|string`
 - Default: `false`
 
-Whether to enable placeholder loading for asynchronous loading content. if it's a stirng, it should be the name of your custom loading component.
+Whether to enable placeholder loading for asynchronous loading content. if it's a string, it should be the name of your custom loading component.
 
 ### cache