Summary:
The debug is supposed to print out two keys to show the value mismatch, which was compared just a few lines above.

However, the actual print-out is the same values (so they obviously won't be mismatched)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/6587

Reviewed By: riversand963

Differential Revision: D23025279

Pulled By: ajkr

fbshipit-source-id: 4c6c35bc60b273f13c08b5464b6f690d8a5cfe41