doc: fixed the link to nginx debug logging doc.

Author: agentzh

README.markdown

@@ -986,7 +986,7 @@ server {
 Trouble Shooting
 ================
 
-To debug issues, you should always check your Nginx `error.log` file first. If no error messages are printed, you need to enable the Nginx debugging logs to get more details, as explained in [[Debugging#Debugging_log]].
+To debug issues, you should always check your Nginx `error.log` file first. If no error messages are printed, you need to enable the Nginx debugging logs to get more details, as explained in [debugging log](http://nginx.org/en/docs/debugging_log.html).
 
 Several common pitfalls for beginners:
 

doc/HttpSRCacheModule.wiki

@@ -810,7 +810,7 @@ This variable was first introduced in the <code>v0.14</code> release.
 
 = Trouble Shooting =
 
-To debug issues, you should always check your Nginx <code>error.log</code> file first. If no error messages are printed, you need to enable the Nginx debugging logs to get more details, as explained in [[Debugging#Debugging_log]].
+To debug issues, you should always check your Nginx <code>error.log</code> file first. If no error messages are printed, you need to enable the Nginx debugging logs to get more details, as explained in [http://nginx.org/en/docs/debugging_log.html debugging log].
 
 Several common pitfalls for beginners: