Fixes event term for pushing root by setting velocity

[Mayankm96]

Description

Previously, the event term set the current root velocity to the sampled values. However, this isn't the same as "pushing" the root. This MR fixes this issue by changing the operation inside.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there