runtime(doc): correct the :public example (#13795)

The current example is no longer supported (E1331), as of
patch 9.0.2167.

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index a7ba2cd..d8d4614 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -1,4 +1,4 @@
-*vim9class.txt*	For Vim version 9.0.  Last change: 2023 Dec 25
+*vim9class.txt*	For Vim version 9.0.  Last change: 2023 Dec 28
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -144,8 +144,8 @@
 If you don't care about side effects and want to allow the object variable to
 be changed at any time, you can make it public: >
 
-	public this.lnum: number
-	public this.col: number
+	public var lnum: number
+	public var col: number
 
 Now you don't need the SetLnum(), SetCol() and SetPosition() methods, setting
 "pos.lnum" directly above will no longer give an error.