RISC-V: Documentation: Add enough title underlines to CMODX

RISC-V: Documentation: Add enough title underlines to CMODX

This reports as a warning in linux-next along the lines of

    Documentation/arch/riscv/cmodx.rst:14: WARNING: Title underline too short.

    CMODX in the Kernel Space
    --------------------- [docutils]
    Documentation/arch/riscv/cmodx.rst:43: WARNING: Title underline too short.

    CMODX in the User Space
    --------------------- [docutils]
    Documentation/arch/riscv/cmodx.rst:43: WARNING: Title underline too short.

    CMODX in the User Space
    --------------------- [docutils]

Link: https://lore.kernel.org/all/20250603154544.1602a8b5@canb.auug.org.au/
Fixes: 0e07200b2af6 ("riscv: Documentation: add a description about dynamic ftrace")
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20250603172856.49925-1-palmer@dabbelt.com
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>