chore(php): add leading slash to avoid import (#4725)

Author: caugner

src/includes/enriching-events/breadcrumbs/breadcrumbs-example/php.mdx

@@ -1,8 +1,8 @@
 ```php
 \Sentry\addBreadcrumb(
     new \Sentry\Breadcrumb(
-        Sentry\Breadcrumb::LEVEL_INFO,
-        Sentry\Breadcrumb::TYPE_DEFAULT,
+        \Sentry\Breadcrumb::LEVEL_INFO,
+        \Sentry\Breadcrumb::TYPE_DEFAULT,
         'auth',                // category
         'Authenticated user',  // message (optional)
         ['user_id' => $userId] // data (optional)