Add missing space in event_source GUC description.

This has apparently been wrong since event_source was added.

Alexander Lakhin

src/backend/utils/misc/guc.c

@@ -2843,7 +2843,7 @@ static struct config_string ConfigureNamesString[] =
 
 	{
 		{"event_source", PGC_POSTMASTER, LOGGING_WHERE,
-			gettext_noop("Sets the application name used to identify"
+			gettext_noop("Sets the application name used to identify "
 						 "PostgreSQL messages in the event log."),
 			NULL
 		},