test: fix typo in comment in threadstest.c

Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15278)

test/threadstest.c

@@ -366,7 +366,7 @@ static void thread_provider_load_unload(void)
  * Test 2: Simple fetch worker
  * Test 3: Worker downgrading a shared EVP_PKEY
  * Test 4: Worker using a shared EVP_PKEY
- * Test 5: Workder loading and unloading a provider
+ * Test 5: Worker loading and unloading a provider
  */
 static int test_multi(int idx)
 {