Fixed minor typo in comment for MergeOperator::FullMergeV2() (#6759)

Summary:
Fixed minor typo in comment for FullMergeV2().
Last operand up to snapshot should be +4 instead of +3.

Signed-off-by: Albert Hse-Lin Chen <hselin@kalista.io>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6759

Reviewed By: cheng-chang

Differential Revision: D21260295

Pulled By: zhichao-cao

fbshipit-source-id: cc942306f246c8606538feb30bfdf6df9fb6c54e

include/rocksdb/merge_operator.h

@@ -125,7 +125,7 @@ class MergeOperator {
   // In the example above, Get(K) operation will call FullMerge with a base
   // value of 2 and operands [+1, +2]. Compaction process might decide to
   // collapse the beginning of the history up to the snapshot by performing
-  // full Merge with base value of 0 and operands [+1, +2, +7, +3].
+  // full Merge with base value of 0 and operands [+1, +2, +7, +4].
   virtual bool FullMergeV2(const MergeOperationInput& merge_in,
                            MergeOperationOutput* merge_out) const;