"Creating MIDI Files" doc is wrong

[James Harkins]

I suppose this must have been reported at some point, but, LP 2.18.2 (stable):

http://lilypond.org/doc/v2.18/Documentation/notation/creating-midi-files

```
To create a MIDI output file from a LilyPond file, insert a \midi block inside 
a \score block;

    \score {
      …music…
      \layout { }
      \midi { }
    }
```

However, the LP version installed from Ubuntu 16.04 packages requires this:

    \layout { }
    \midi { }
    \score {
      …music…
    }

\layout{} or \midi{} inside the \score causes a syntax error:

```
Starting lilypond 2.18.2 [climax-test.ly]...
Processing `/home/dlm/share/SC/scd/khaen/tests/climax-test.ly'
Parsing...
/home/dlm/share/SC/scd/khaen/tests/climax-test.ly:90:3: error: syntax error, 
unexpected \layout
  
  \layout {}
/home/dlm/share/SC/scd/khaen/tests/climax-test.ly:4:1: error: errors found, 
ignoring music expression

\new Score {
```

$ lilypond --version
GNU LilyPond 2.18.2

hjh