fix(lmweight):Fixed upperbound wrong bounds formu

Upper bound formula for TWO_STAGE and DIRICHLET should have doc_length_upper_bound
in numerator rather than doc_length_lower_bound. Since it sum was exceeding the bound.
Replace lowerbound in numerator with upper bound.
If it's more readable, a local variable is certainly fine.

Compilers can optimise away local variables easily, but an extra member in a class is much harder for to optimise away.