fixed the link to ngx_openresty.

README.markdown

@@ -1262,7 +1262,7 @@ Installation
 
 You're recommended to install this module as well as the Lua interpreter or LuaJIT 2.0 (with many other good stuffs) via the ngx_openresty bundle:
 
-    https://github.com/agentzh/ngx_openresty
+<https://github.com/agentzh/ngx_openresty>
 
 The installation steps are usually as simple as ./configure && make && make install