Update documentation to capture name change from rippled to xrpld

[amarantha-k]

PR XRPLF/rippled#5983 renames the rippled binary to xrpld. As noted in its description, in addition to renaming ripple(d) to xrpl(d), the PR also creates a symlink so rippled redirects to xrpld. The plan is to remove this symlink in about 6 months to give everyone plenty of time to update these automations and scripts to reference xrpld instead.

This ticket tracks the documentation update to reflect this updated name for the executable.