Fixes instructions on importing ANYmal URDF in docs

[Mayankm96]

Description

Previously, the ANYmal URDF importer example was setting the stiffness and damping to new defaults (100.0 and 0.1 respectively). However, this is undesirable as we want to import the asset with no stiffness and damping (as we do effort control). This MR corrects the instructions to convert ANYmal asset.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

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