std.fs.File: fix typo in writeFileAll

lib/std/fs/file.zig

@@ -728,7 +728,7 @@ pub const File = struct {
         }
         var i: usize = 0;
         while (i < trailers.len) {
-            while (amt >= headers[i].iov_len) {
+            while (amt >= trailers[i].iov_len) {
                 amt -= trailers[i].iov_len;
                 i += 1;
                 if (i >= trailers.len) return;