MINOR: Update ORCv1.md: Fix documentation about bitset in bloom filter section (#670)
MINOR: Update ORCv1.md: Fix documentation about bitset in bloom filter section (#670)

The corresponding code is https://github.com/apache/orc/blob/master/c%2B%2B/src/BloomFilter.cc#L47.

The code is correct, but the documentation is not. The code is to use `index % 64` which has the same effect as `index & 0x3f`.
```
  constexpr uint64_t BITS_OF_LONG = 64;
  constexpr uint8_t  SHIFT_6_BITS = 6;
  void BitSet::set(uint64_t index) {
    mData[index >> SHIFT_6_BITS] |= (1ULL << (index % BITS_OF_LONG));
  }
```

The fixes on documentation are
1. There is no need to use `>>>`. We have already flip `combinedHash` if it's negative and `position`  sign bit will be always zero.
2. To set bit we have to mask `position` with 0x3f to keep the least 6 bits.
omalley/orc,omalley/orc,omalley/orc,apache/orc,omalley/orc,apache/orc,apache/orc,apache/orc,omalley/orc,apache/orc,apache/orc,omalley/orc,apache/orc,apache/orc,omalley/orc