Some documents on official site is outdate

For example:

on doc site
https://juliamusic.github.io/MusicXML.jl/index.html

```
# Extracting each instrument information
scprts = doc.scorepartwise.partlist.scoreparts
```

but actually
https://github.com/JuliaMusic/MusicXML.jl/blob/master/docs/src/parsing.md
```
# Extracting each instrument information
scprts = scorepartwise.partlist.scoreparts
```