Fixed typo: T replaced S in non-local flux variable

- Typo was introduced in 2bb51feccae321f65f441a6a848812edc3519b05.
- The typo caused answer changes in all the MOM6 regresion tests
  that use KPP.
Merge pull request #27 from CVMix/bugfix/kpp_T_S_typo

Fixed typo: T replaced S in non-local flux variable