wrong string extraction in python/musicxml.py

[Federico Bruni]

While translating the PO file I saw what I thought it was a typo 
(butthe), but it's something else.

$ git grep -n -B2 -A2 butthe po/lilypond.pot
po/lilypond.pot-847-#: musicxml.py:116
po/lilypond.pot-848-#, python-format
po/lilypond.pot:849:msgid "more than one child of class %s, all butthe 
first will be ignored"
po/lilypond.pot-850-msgstr ""
po/lilypond.pot-851-

$ git grep -n -A2 'all but' python/*
python/musicxml.py:116: warnings.warn(_('more than one child of class 
%s, all but'
python/musicxml.py-117- 'the first will be ignored') % klass.__name__)
python/musicxml.py-118- return cn[0]