FIX: small typo in an example using the "Referer" header

It was reported that an example was manipulating a "Referrer" header instead
of the known "Referer" one. Even if it's an example wich doesn't break things,
the typo can be fixed.

The fix should be backported in 1.4/1.5/1.6 branches.

doc/configuration.txt

@@ -2398,7 +2398,7 @@ capture request header <name> len <length>
   Example:
         capture request header Host len 15
         capture request header X-Forwarded-For len 15
-        capture request header Referrer len 15
+        capture request header Referer len 15
 
   See also : "capture cookie", "capture response header" as well as section 8
              about logging.