Modify "v (list) : state value function" to "V"

[hslyu]

It would better express list as uppercase, since it is well-known nomenclature in large areas of mathematics, computer sciences and most of papers. Also, in the function definition, state value function is defined is uppercase "V", not "v". Need to unify the variable names.

If changed, I'll change all corresponding .ipynb codes in the DP directory.