Update Time.add message about units

[solar05]

Since spec tells that Time.add support :hour and :minute:

@spec add(Calendar.time(), integer(), :hour | :minute | System.time_unit()) :: t()

But actual message is:

Time.add(Time.utc_now(), 2, :minutes)
** (ArgumentError) unsupported time unit. Expected :second, :millisecond, :microsecond, :nanosecond, or a positive integer, got :minutes

After fix:

Time.add(Time.utc_now(), 2, :minutes)
** (ArgumentError) unsupported time unit. Expected :hour, :minute, :second, :millisecond, :microsecond, :nanosecond, or a positive integer, got :minutes

[josevalim]

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