Incorrect example in documentation

[Nate Whetsell]

Hi there,

Thank you for all your work on LilyPond!

The syntax for \revert-ing certain properties appears to have changed after 
LilyPond 2.18, which makes the “Altering the length of beamed stems” example in 
the documentation slightly incorrect. At

http://lilypond.org/doc/v2.18/Documentation/snippets/pitches#pitches-altering-the-length-of-beamed-stems
 
<http://lilypond.org/doc/v2.18/Documentation/snippets/pitches#pitches-altering-the-length-of-beamed-stems>

note that the last six notes have default length. At

http://lilypond.org/doc/v2.22/Documentation/snippets/pitches#pitches-altering-the-length-of-beamed-stems
 
<http://lilypond.org/doc/v2.22/Documentation/snippets/pitches#pitches-altering-the-length-of-beamed-stems>

the stems of the last six notes are still 8.5 staff spaces long. This can be 
fixed by changing the third-to-last line of the example code to

\revert Stem.details.beamed-lengths

For the sake of completeness, here’s an example that illustrates the issue:

```
\version "2.22.0"
{
  \override Stem.details.beamed-lengths = #'(10)
  c'8 8 8 8
  \revert Stem.details
  8 8 8 8 % These stems are still too long.
  \revert Stem.details.beamed-lengths
  8 8 8 8
}
```

All the best,
Nate