Fix incorrect topic name in nav2_velocity_smoother README

[faseelmo]

Issue

The README in nav2_velocity_smoother incorrectly lists the publisher topic as smoothed_cmd_vel, while the actual topic in code is cmd_vel_smoothed.

Description of contribution

• Updated the README to reflect the correct topic name of the Twist message publisher.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists
• Should this be backported to current distributions? If so, tag with backport-*.

[mergify]

@faseelmo, your PR has failed to build. Please check CI outputs and resolve issues.
You may need to rebase or pull in main due to API changes (or your contribution genuinely fails).