[V1][Metrics] Tweak prefix caching note

Co-authored-by: Cody Yu <hao.yu.cody@gmail.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>

Author: markmc

docs/source/design/v1/metrics.md

@@ -279,7 +279,7 @@ every 5 seconds with some key metrics:
   seconds
 - The number of new tokens generated per second over the past 5
   seconds
-- The prefix cache hit rate over the most recent 1 queries
+- The prefix cache hit rate over the most recent 1k kv-cache block queries
 
 ### Metrics Publishing - Prometheus