Fixed error message.

src/http/ngx_http.c

diff --git a/src/http/ngx_http.c b/src/http/ngx_http.c
index e809045..4f0b79d 100644
--- a/src/http/ngx_http.c
+++ b/src/http/ngx_http.c
@@ -1281,7 +1281,7 @@
 
             if (addr[i].opt.set) {
                 ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
-                        "a duplicate listen options for %s", addr[i].opt.addr);
+                        "duplicate listen options for %s", addr[i].opt.addr);
                 return NGX_ERROR;
             }