readme: fix syntax for log_by_lua_file

Author: hypebeast

README.md

@@ -223,7 +223,7 @@ This directive was first introduced in the `v0.0.3` release.
 log_by_lua_file
 --------------------
 
-**syntax:** *preread_by_lua_file <path-to-lua-script-file>*
+**syntax:** *log_by_lua_file <path-to-lua-script-file>*
 
 **context:** *stream, server*