Fully qualify SlashCommandBuilder namespace (#2534)

Update to make sure its cleear which SlashCommandBuilder is used.

docs/guides/int_basics/application-commands/slash-commands/parameters.md

@@ -45,7 +45,7 @@ public async Task Client_Ready()
 {
     ulong guildId = 848176216011046962;
 
-    var guildCommand = new SlashCommandBuilder()
+    var guildCommand = new Discord.SlashCommandBuilder()
         .WithName("list-roles")
         .WithDescription("Lists all roles of a user.")
         .AddOption("user", ApplicationCommandOptionType.User, "The users whos roles you want to be listed", isRequired: true);