Documentation/RCU: Fix nested inline markup

To avoid the ``foo`` markup inside the `bar`__ hyperlink marker,
use the "replace" directive [1].

This should restore the intended appearance of the link.

Tested with sphinx versions 1.7.9 and 2.4.4.

[1]: https://docutils.sourceforge.io/docs/ref/rst/directives.html#replace



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