According to context,the size should be 16 rather than 64
FIXED  a typo  more thank  should be more than
refer to updateLRUClock's comment REDIS_LRU_CLOCK_MAX is 22 bits,but #define REDIS_LRU_CLOCK_MAX ((1<<21)-1) only 21 bits
Merge pull request #1576 from Hailei/fix-lruidletime-comment

Fix REDIS_LRU_CLOCK_MAX's value