Fix Invalid Syntax in Documentation

[thiagopromano]

The current example gives the following error:

** (FunctionClauseError) no function clause matching in Postgrex.DefaultTypes.encode_params/3    
    
    The following arguments were given to Postgrex.DefaultTypes.encode_params/3:
    
        # 1
        "SET lock_timeout TO '10s'"
    
        # 2
        []
    
        # 3
        []
    
    Attempted function clauses (showing 3 out of 3):
    
        defp encode_params([param | params], [type | types], encoded)
        defp encode_params([], [], encoded)
        defp encode_params(params, _, _) when is_list(params)

[josevalim]

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