Corrected the directive name in the ngx_mail_auth_http_module error message.

src/mail/ngx_mail_auth_http_module.c

diff --git a/src/mail/ngx_mail_auth_http_module.c b/src/mail/ngx_mail_auth_http_module.c
index c23c4ba..d2bd4ad 100644
--- a/src/mail/ngx_mail_auth_http_module.c
+++ b/src/mail/ngx_mail_auth_http_module.c
@@ -1332,7 +1332,7 @@
 
         if (conf->peer == NULL) {
             ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
-                          "no \"http_auth\" is defined for server in %s:%ui",
+                          "no \"auth_http\" is defined for server in %s:%ui",
                           conf->file, conf->line);
 
             return NGX_CONF_ERROR;