Tweak a comment in R_Addition_MultipleBond/training

When this is exported to RMG-Java style database, it became ambiguous.
(It said "THIS is the full rate not the per-site rate" but that was 
no longer true, because the comment was re-used on a rate-rule which
has been translated into a per-site basis.)
exportOldDatabase.py removes heteroatoms from thermo libraries.

Some thermo libraries contain nitrogen atoms, which confuses RMG-Java.
This prunes those species, so the exported database can run in RMG-Java.
Currently it only allows C,H,O and S through.