Fixed a doc

Signed-off-by: Vishal Rana <vr@labstack.com>

Author: vishr

website/content/guide/customization.md

@@ -25,7 +25,7 @@ and message `HTTPError.Message`.
 
 Enables/disables debug mode.
 
-## Log prefix
+### Log prefix
 
 `echo#SetLogPrefix(prefix string)`