Fix the retailer-drug-balance function

[blessingblockchain]

The smart contract code appears to be mostly correct. However, there is one issue in the retailerDrugBalance function. Currently, the function is returning the balance of a specific drug for a retailer. The issue is that it is using retailer as the parameter instead of retailer_id[_retailercount]

hi @chiranjeev13 assign me this issue