DOC: management: rename some last occurences from domain "dns" to "resolvers"

DOC: management: rename some last occurences from domain "dns" to "resolvers"

This is a complementary patch to cf913c2f9 ("DOC: management: rename show
stats domain cli "dns" to "resolvers"). The doc still refered to the
legacy "dns" domain filter for stat command. Let's rename those occurences
to "resolvers".

It may be backported to all stable versions.

(cherry picked from commit 4c3eb60e7019d12734501ceb9358d2714bce8922)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit b794eb91aa57be36c90439a7f1574a757ad84e9d)
Signed-off-by: Willy Tarreau <w@1wt.eu>