DOC: Fix documentation about the cli command to get resolver stats

In the management guide, this command was still referenced as "show stat
resolvers" instead of "show resolvers". The cli command was fixed by the commit
ff88efbd7 ("BUG/MINOR: dns: Fix CLI keyword declaration").

This patch fixes the issue #296. It can be backported as fas as 1.7.
diff --git a/doc/management.txt b/doc/management.txt
index 9973747..73fda89 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -2440,7 +2440,7 @@
   $ echo "show stat json" | socat /var/run/haproxy.sock stdio | \
     python -m json.tool
 
-show stat resolvers [<resolvers section id>]
+show resolvers [<resolvers section id>]
   Dump statistics for the given resolvers section, or all resolvers sections
   if no section is supplied.