Small error in equation in LookupTable part of `doc/convolution.md` (#831)
Small error in equation in LookupTable part of `doc/convolution.md` (#831)

* Corrected small error in convolution.md
apaszke/nn,sagarwaghmare69/nn,eriche2016/nn,diz-vara/nn,joeyhng/nn,kmul00/nn,nicholas-leonard/nn,colesbury/nn