Update process filter example (Bug#35044)

* doc/lispref/processes.texi (Filter Functions): Use
insert-before-markers in the "ordinary" filter example, like
internal-default-process-filter does.

Author: npostavs

doc/lispref/processes.texi

@@ -1643,7 +1643,7 @@ how to do these things:
         (save-excursion
           ;; @r{Insert the text, advancing the process marker.}
           (goto-char (process-mark proc))
-          (insert string)
+          (insert-before-markers string)
           (set-marker (process-mark proc) (point)))
         (if moving (goto-char (process-mark proc)))))))
 @end group