Fixed typo in comment in version_set.h.
Fixed typo in comment in version_set.h.

Flagged by presubmit check.

PiperOrigin-RevId: 243118632

likema/leveldb,dwing4g/leveldb,google/leveldb,cmumford/leveldb,fastogt/leveldb,matehat/leveldb,google/leveldb,bitcoin-core/leveldb,bitcoin-core/leveldb,likema/leveldb,dwing4g/leveldb,dwing4g/leveldb,bitcoin-core/leveldb,cmumford/leveldb,matehat/leveldb,matehat/leveldb,fastogt/leveldb