Update v6 migration guide for shouldForwardProp (#939)

* fix: update broken link

* docs: more detailed code example for v6 prop forwarding migration

* fix: update code indentation to match existing style

---------

Co-authored-by: Ben Cole <bencole@twitch.tv>

sections/faqs/migration-v6.mdx

@@ -34,11 +34,21 @@ import { StyleSheetManager } from 'styled-components';
 
 function MyApp() {
     return (
-        <StyleSheetManager shouldForwardProp={isPropValid}>
+        <StyleSheetManager shouldForwardProp={shouldForwardProp}>
             {/* other providers or your application's JSX */}
         </StyleSheetManager>
     )
 }
+
+// This implements the default behavior from styled-components v5
+function shouldForwardProp(propName, target) {
+    if (typeof target === "string") {
+        // For HTML elements, forward the prop if it is a valid HTML attribute
+        return isPropValid(propName);
+    }
+    // For other elements, forward all props
+    return true;
+}
 ```
 
 ### Vendor prefixes are omitted by default