docs: maybe math mode more appropriate here?
docs: maybe math mode more appropriate here?

`S^d(n,k)` isn't code so makes sense to typeset as mathematics.  But
`n`, `k` and `d` here also refer to input parameters to the Python
function.  There are ambigious cases around: would be a good to have
a style guide that deals with this...

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy