selftests/bpf: Fix spelling mistake "get_signaure_size" -> "get_signature_size"

There is a spelling mistake in an ASSERT_GT message. Fix it.
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231204093940.2611954-1-colin.i.king@gmail.com

tools/testing/selftests/bpf/prog_tests/verify_pkcs7_sig.c

@@ -498,7 +498,7 @@ static void test_pkcs7_sig_fsverity(void)
 	if (!ASSERT_OK_PTR(skel, "test_sig_in_xattr__open"))
 		goto out;
 	ret = get_signature_size(sig_path);
-	if (!ASSERT_GT(ret, 0, "get_signaure_size"))
+	if (!ASSERT_GT(ret, 0, "get_signature_size"))
 		goto out;
 	skel->bss->sig_size = ret;
 	skel->bss->user_keyring_serial = syscall(__NR_request_key, "keyring",