Align PHP example keys with other User reserved profile properties (#1867)

All other identify code examples at time of writing use $name.

The list of reserved profile properties includes $name, $first_name, $last_name:
https://docs.mixpanel.com/docs/data-structure/property-reference/reserved-properties#reserved-profile-properties

Co-authored-by: myronkaifung <[email protected]>

Author: johnkary

pages/docs/quickstart/identify-users.mdx

@@ -51,8 +51,7 @@ You may want to disable ip geolocation when using a server-side SDK. You can lea
 ```shell php
 // create/update a profile for user id 12345
 $mp->people->set('USER_ID', array(
-    '$first_name'       => "John",
-    '$last_name'        => "Doe",
+    '$name'             => "John Doe",
     '$email'            => "[email protected]",
     '$phone'            => "5555555555",
     "Favorite Color"    => "red"