Fix `abjad.beam(..., stemlet_length=None)` formatting

[trevorbaca]

EXAMPLE.

>>> voice = abjad.Voice("c'8 d' e' f'")
>>> abjad.beam(voice[:], stemlet_length=0.75)
>>> string = abjad.lilypond(voice)

OLD: The position of the \revert string is correct:

>>> print(string)
\new Voice
{
    \override Staff.Stem.stemlet-length = 0.75
    c'8
    [
    d'8
    e'8
    \revert Staff.Stem.stemlet-length
    f'8
    ]
}

NEW: the position of the \revert string is correct:

>>> print(string)
\new Voice
{
    \override Staff.Stem.stemlet-length = 0.75
    c'8
    [
    d'8
    e'8
    f'8
    ]
    \revert Staff.Stem.stemlet-length
}