runtime(doc): Fix erroneous *roff syntax examples

runtime(doc): Fix erroneous *roff syntax examples

Problem dates back at least to commit 071d4279d6, Vim 7.0001 (2004).

The examples shown were of special character interpolation syntax, not
("number") register interpolation syntax.

Sources:
* https://www.gnu.org/software/groff/manual/groff.html.node/Built_002din-Registers.html
* https://github.com/mamccollum/troff-resources/blob/main/cstr-54.pdf

closes: #19243

Signed-off-by: G. Branden Robinson <g.branden.robinson@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>