Fix "foreach"/"for" related paragraph in mscore3.txt

As @dmitrio95 pointed in https://github.com/musescore/MuseScore/pull/4511, there is possible mistake in https://github.com/musescore/MuseScore/blob/master/mscore3.txt#L148-L152.

I did face problems related to replacing foreach with for. Afair, foreach always creates a copy of the container and operates on the latter, and for does not.