riscv: Docs: fix unmatched quote warning

riscv: Docs: fix unmatched quote warning

'make htmldocs' complains about ``prctrl` -- so add a second '`' to
avoid the warning.

Documentation/arch/riscv/zicfilp.rst:79: WARNING: Inline literal start-string without end-string. [docutils]

Fixes: 08ee1559052b ("prctl: cfi: change the branch landing pad prctl()s to be more descriptive")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260406232304.1892528-1-rdunlap@infradead.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>