ja: Fix typo of Big-O notation in KVS section

Created by: Wintus

This fix on the translation is a correction based on the original English version: https://github.com/donnemartin/system-design-primer/blob/33431e61a9e6a636c902298fe3629da3a34c3c66/README.md#L1003

No change on the original version.

Ref: https://github.com/donnemartin/system-design-primer/blob/master/CONTRIBUTING.md#changes-to-translations