fix misspelling in documentation conf.py
fix misspelling in documentation conf.py

mathjax cdn url was not being set to https because of this.

zakandrewking/cobrapy,zakandrewking/cobrapy,jerkos/cobrapy,jeicher/cobrapy,aebrahim/cobrapy,JuBra/cobrapy,JuBra/cobrapy,aebrahim/cobrapy,jeicher/cobrapy,jerkos/cobrapy