correct typo in adjlist.py: check_multiplicity

if multiplicity is even, then we have a problem.
charge is calculated, NOT considering maximum valence of period

the charge of atom can be calculated by considering its
- theoretical valency (eg. carbon = 4, oxygen = 6)
- the number of unpaired electrons
- the number of lone electron pairs
- the bond order (connectivity with other atoms)

the maximum valency of 2 or 8 does not come into this equation.
correct check of multiplicity for doublets

also, added some more info on what we exactly check