Update take docstring to match the behaviour

[nwolverson]

This is a documentation-only change to reconcile the documentation of Data.String.CodePoints.take with the behaviour. The behaviour of take is to return the first N characters of the string, or the entire input string if the length is less than N. The falllback and direct implementations appear to agree on this. The tests verify this behaviour (str length is 7):

  assertEqual
    { actual: SCP.take 8 str
    , expected: str
    }