Update Emby.Server.Implementations/Library/UserManager.cs

Co-Authored-By: LogicalPhallacy <44458166+LogicalPhallacy@users.noreply.github.com>

Emby.Server.Implementations/Library/UserManager.cs

@@ -218,7 +218,7 @@ namespace Emby.Server.Implementations.Library
         {
             // This is some regex that matches only on unicode "word" characters, as well as -, _ and @
             // In theory this will cut out most if not all 'control' characters which should help minimize any weirdness
-            // Usernames can contain letters (a-z + whatever else unicode is cool with), numbers (0-9), dashes (-), underscores (_), apostrophes ('), and periods (.)
+             // Usernames can contain letters (a-z + whatever else unicode is cool with), numbers (0-9), at-signs (@), dashes (-), underscores (_), apostrophes ('), and periods (.)
             return Regex.IsMatch(username, @"^[\w\-'._@]*$");
         }