Update default log file size in doc.
Update default log file size in doc.

The default size was changed in #f779e7a5 but the documentation was
never updated.

This fixes #566 reported on GitHub.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194547959

cmumford/leveldb,matehat/leveldb,fuchsia-mirror/third_party-leveldb,luke-jr/leveldb,likema/leveldb,matehat/leveldb,cmumford/leveldb,bitcoin-core/leveldb,dwing4g/leveldb,dwing4g/leveldb,matehat/leveldb,bitcoin-core/leveldb,yuwolf/leveldb,dwing4g/leveldb,google/leveldb,google/leveldb,fastogt/leveldb,fuchsia-mirror/third_party-leveldb,yuwolf/leveldb,fastogt/leveldb,bitcoin-core/leveldb,likema/leveldb,luke-jr/leveldb,yuwolf/leveldb