rv: Fix documentation reference in da_monitor.h

rv: Fix documentation reference in da_monitor.h

Update documentation reference to reflect the file rename.
Monitor synthesis documentation was renamed in commit f40a7c060207
("Documentation/rv: Prepare monitor synthesis document for LTL inclusion")
from da_monitor_synthesis.rst to monitor_synthesis.rst.

Signed-off-by: Shubham Sharma <slopixelz@gmail.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20251230075337.11993-1-slopixelz@gmail.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>