Doc bug: Span_dynamic_performer

[Daniel Johnson]

In section 10.3.1 of the Fine Manual (for both 2.10 and 2.11), we see the
following:

To remove dynamics from the MIDI output, insert the following lines in the
\midi{} section.

\midi {
  ...
  \context {
    \Voice
    \remove "Dynamic_performer"
    \remove "Span_dynamic_performer"
  }
}

Span_dynamic_performer no longer exists; its functions are now part of
Dynamic_performer.