Fix invalid Luau syntax in matchmaking/attributes-and-signals.md (#1271)

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Author: ari-party

content/en-us/matchmaking/attributes-and-signals.md

@@ -221,7 +221,7 @@ end
 ```
 
 ```lua title="Server attribute example"
-local MatchmakingService = game::GetService("MatchmakingService")
+local MatchmakingService = game:GetService("MatchmakingService")
 local RunService = game:GetService("RunService")
 
 if RunService:IsStudio() then