fix dissect doc "ip" --> "clientip"
Forward port of
@@ -170,7 +170,7 @@ Named skip key modifier example
| *Pattern* | `%{clientip} %{?ident} %{?auth} [%{@timestamp}]`
| *Input* | 1.2.3.4 - - [30/Apr/1998:22:00:52 +0000]
| *Result* a|
-* ip = 1.2.3.4
+* clientip = 1.2.3.4
* @timestamp = 30/Apr/1998:22:00:52 +0000
|======
