DOC: management: fix typo on "cache_lookups" stats output

The trailing "s" was missing.
diff --git a/doc/management.txt b/doc/management.txt
index 1a2e132..ffd3489 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1101,7 +1101,7 @@
  83: wrew [LFBS]: cumulative number of failed header rewriting warnings
  84: connect [..BS]: cumulative number of connection establishment attempts
  85: reuse [..BS]: cumulative number of connection reuses
- 86: cache_lookup [.FB.]: cumulative number of cache lookups
+ 86: cache_lookups [.FB.]: cumulative number of cache lookups
  87: cache_hits [.FB.]: cumulative number of cache hits