test: fix stderr Buffer assertion in exec encoding test

Signed-off-by: greenhead <greenheadhq@gmail.com>
PR-URL: #66008
Refs: #10919
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Xuguang Mei <meixuguang@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>

Author: greenheadHQ

test/parallel/test-child-process-exec-encoding.js

@@ -41,7 +41,7 @@ if (process.argv[2] === 'child') {
   [undefined, null, 'buffer', 'invalid'].forEach((encoding) => {
     run({ encoding }, common.mustCall((stdout, stderr) => {
       assert(stdout instanceof Buffer);
-      assert(stdout instanceof Buffer);
+      assert(stderr instanceof Buffer);
       assert.strictEqual(stdout.toString(), expectedStdout);
       assert.strictEqual(stderr.toString(), expectedStderr);
     }));