doc modification

[Pierre Perol-Schneider]

Hi,

This thread :
http://lilypond.1069038.n5.nabble.com/Positioning-spanner-at-different-height-after-line-break-td163618.html

made me think that maybe it would be good to find another example for this
part :
http://www.lilypond.org/doc/v2.18/Documentation/extending/difficult-tweaks.html

since it's now easy to tweak with the \alterBroken command.

Cheers,
Pierre