Enumeration#maxId: fix documentation to reflect reality

maxId is not really the "maximum id", but rather one past the max:

scala> object Foo extends Enumeration {
     |   val x = Value
     |   val y = Value
     | }
defined module Foo

scala> Foo.maxId
res0: Int = 2

scala> Foo(2)
java.util.NoSuchElementException: key not found: 2
	at scala.collection.MapLike$class.default(MapLike.scala:225)
        ...

scala> Foo(1)
res2: Foo.Value = y

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