server: fix += -> + typo

We don't want to free inside the range...

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index c1a2824..553eb82 100644
--- a/server.c
+++ b/server.c
@@ -750,7 +750,7 @@
 	if (!fio_server_arg)
 		return 0;
 	if (!strncmp(fio_server_arg, "sock:", 5)) {
-		bind_sock = fio_server_arg += 5;
+		bind_sock = fio_server_arg + 5;
 		return 0;
 	} else {
 		char *host = fio_server_arg;