update doc to reflect setTheme function change (#32490)

Author: marchbox

change/@fluentui-web-components-c0548477-ac6b-4e98-9f54-66384b5a041d.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "docs: reflect setTheme change",
+  "packageName": "@fluentui/web-components",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

packages/web-components/src/_docs/developer/theming.stories.mdx

@@ -29,15 +29,15 @@ import { webLightTheme } from '@fluentui/tokens';
 setTheme(webLightTheme);
 ```
 
-If you need to set token values for an element which is not the document body, you can use `setThemeFor` and pass an element followed by the theme.
+If you need to set token values for an element which is not the document body, you can pass in the element as the second argument in `setTheme`.
 
 ```js
-import { setThemeFor } from '@fluentui/web-components';
-import { webLightTheme } from '@fluentui/tokens';
+import { setTheme } from '@fluentui/web-components';
+import { webDarkTheme } from '@fluentui/tokens';
 
 const el = document.getElementById('my-element');
 
-setThemeFor(el, webLightTheme);
+setTheme(webDarkTheme, el);
 ```
 
 ### Do not use CSS variables directly