Fixed writev() debug log message in ngx_darwin_sendfile_chain().

src/os/unix/ngx_darwin_sendfile_chain.c

diff --git a/src/os/unix/ngx_darwin_sendfile_chain.c b/src/os/unix/ngx_darwin_sendfile_chain.c
index ee44e1d..068add9 100644
--- a/src/os/unix/ngx_darwin_sendfile_chain.c
+++ b/src/os/unix/ngx_darwin_sendfile_chain.c
@@ -285,7 +285,7 @@
             rc = writev(c->fd, header.elts, header.nelts);
 
             ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
-                           "writev: %d of %uz", rc, send);
+                           "writev: %d of %O", rc, header_size);
 
             if (rc == -1) {
                 err = ngx_errno;