fix(docs): fix wrong method name in quickstart.md (#1729)

Hi! There is a mistake in doc's, you used the wrong method, method `slice` will be correct.

website/docs/en-US/quickstart.md

@@ -129,7 +129,7 @@ export default defineConfig({
       libs: [{
         libraryName: 'element-plus',
         resolveStyle: (name) => {
-          name = name.splice(3)
+          name = name.slice(3)
           return `element-plus/packages/theme-chalk/src/${name}.scss`;
         },
         resolveComponent: (name) => {