Fix problem of molecule 847

[FanwangM]

The molecule 847 - 3-methylhexane (SMILES C=C([CH]CC)CC) is problematic as there is a carbon atom with unusual valence and the SMILES does not match SMILES for 3-methylhexane. This should be https://pubchem.ncbi.nlm.nih.gov/compound/11507.

Credit goes to Dr. Andrea Mauri from Alvascience.