runtime(doc): improve documentation style in editing.txt

runtime(doc): improve documentation style in editing.txt

Usually, Vim's document provides example code after explanations.
However some part of the editing.txt doesn't follow the style, therefore
this commit modifies it so that it follows the usual style.

closes: #17607

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: mityu <mityu.mail@gmail.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>