doc: RCU: Avoid 'Symbol' font-family in SVG figures

On Ubuntu Focal, strings in some of SVG files under
Documentation/RCU/Design can not be rendered properly when
converted to PDF.

Ubuntu releases since Focal and Debian bullseye have trouble
with "Symbol" font-family in SVG files.

As those strings are mostly API names such as "READ_ONCE()",
"WRITE_ONCE(), "rcu_read_lock()", and so on, using a generic
monospace font-family should be a good alternative.

Substitute the font-family name by a simple sed pattern:

    's/Symbol/monospace/g'

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>