MINOR: doc: fix use-server example (imap vs mail)

Another minor doc issue in the use-server example, use-server refers
to server "imap", but the server below is actually called "mail".

Renames the server from "mail" to "imap".
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 3a02840..09aaf1d 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -10069,7 +10069,7 @@
      use-server mail if { req_ssl_sni -i mail.example.com }
      server     mail 192.168.0.1:587 weight 0
      use-server imap if { req_ssl_sni -i imap.example.com }
-     server     mail 192.168.0.1:993 weight 0
+     server     imap 192.168.0.1:993 weight 0
      # all the rest is forwarded to this server
      server  default 192.168.0.2:443 check