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 <zhangchujun@cmss.chinamobile.com>

Link: https://patch.msgid.link/20251106055819.1996-1-zhangchujun@cmss.chinamobile.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: Zhang Chujun

tools/testing/selftests/alsa/conf.c

@@ -448,7 +448,7 @@ int conf_get_bool(snd_config_t *root, const char *key1, const char *key2, int de
 		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;
 }