#10459: docs: replace --to with --target in message send example

Fixes the README command example to match current CLI flags. - `openclaw message send --to ...` -> `openclaw message send --target ...` Closes #10458. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Updates the README “message send” quick-start example to use the current `--target` flag instead of the deprecated `--to`. - Keeps the rest of the quick-start flow unchanged (onboard → gateway → send message → agent). - Aligns documentation with the CLI option naming used in `src/cli/program/register.message.ts` / related message command wiring. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge; it is a small, correct documentation-only flag rename. - The change is limited to a single README command example and matches the CLI’s existing `--target` option usage in the codebase; no functional code paths are modified. - README.md <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->