fix a typo (of a potential vi user)

Summary: Closes https://github.com/facebook/rocksdb/pull/3481

Differential Revision: D6939089

Pulled By: siying

fbshipit-source-id: ccce3ae10cc5ff50a74b85804afd044b21a3c3e2

include/rocksdb/options.h

@@ -560,7 +560,7 @@ struct DBOptions {
   //    then WAL_size_limit_MB, they will be deleted starting with the
   //    earliest until size_limit is met. All empty files will be deleted.
   // 3. If WAL_ttl_seconds is not 0 and WAL_size_limit_MB is 0, then
-  //    WAL files will be checked every WAL_ttl_secondsi / 2 and those that
+  //    WAL files will be checked every WAL_ttl_seconds / 2 and those that
   //    are older than WAL_ttl_seconds will be deleted.
   // 4. If both are not 0, WAL files will be checked every 10 min and both
   //    checks will be performed with ttl being first.