selftest/alsa: correct grammar in conf_get_bool error string

The phrase "an bool" is grammatically incorrect; it should be
"a bool".

Signed-off-by: Zhang Chujun <[e-mail]>

Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

tools/testing/selftests/alsa/conf.c

···
 		ksft_exit_fail_msg("key '%s'.'%s' search error: %s\n", key1, key2, snd_strerror(ret));
 	ret = snd_config_get_bool(cfg);
 	if (ret < 0)
-		ksft_exit_fail_msg("key '%s'.'%s' is not an bool\n", key1, key2);
+		ksft_exit_fail_msg("key '%s'.'%s' is not a bool\n", key1, key2);
 	return !!ret;
 }