Fix typo in 15to16 upgrading notes

Author: TysonAndre

Usage Docs/15to16.md

@@ -6,7 +6,7 @@ This guide will help you navigate the changes that were introduced in v16.
 
 ## Client supports multiple socket.io versions
 
-The client now supports socket.io 3 servers. This is mostly a transparent change, however if your sever
+The client now supports socket.io 3 servers. This is mostly a transparent change, however if your server
 is socket.io 2, you must send `.version(.two)` as an option to the manager.
 
 ```swift