libplugin: fix LOG_TRACE for plugins

A log event LOG_TRACE submitted by a plugin was being logged as
**BROKEN** by lightningd before this commit.

Changelog-Fixed: plugins can now log events under the LOG_TRACE flag.

Signed-off-by: Lagrang3 <lagrang3@protonmail.com>

lightningd/plugin.c

@@ -498,7 +498,7 @@ static const char *plugin_log_handle(struct plugin *plugin,
 		 || level == LOG_IO_OUT) {
 		return tal_fmt(plugin,
 			       "Unknown log-level %.*s, valid values are "
-			       "\"debug\", \"info\", \"warn\", or \"error\".",
+			       "\"trace\", \"debug\", \"info\", \"warn\", or \"error\".",
 			       json_tok_full_len(leveltok),
 			       json_tok_full(plugin->buffer, leveltok));
 	}