Return type inconsistency

### Issue

I've identified two issues in the bit_manipulation/count_of_set_bits.cpp repository file. The first is a return type mismatch: the function returns an int, but the documented type is uint64_t. The second is an erroneous documentation link, which directs to the count-set-bits problem instead of the correct count-bits-flip problem.
Can you assign me this issue i can fix this.
