chore: update description for --assign-agent option

[dlwldnjs1009]

Description:

What changes:
This Pull Request corrects the description of the --assign-agent option in the Backend.AI CLI. The previous description suggested that this option is used to display a mapping list of tuples which mapped containers with agents.

Justification:
The --assign-agent option is actually used to assign specific agents to a session. This functionality is only applicable when the user role is a Super Admin. This discrepancy between the actual functionality and the description in the code can lead to confusion for users and developers.

Before:

@click.option(
    "--assign-agent",
    default=None,
    type=list_expr,
    help=(
        "Show mapping list of tuple which mapped containers with agent. "
        "When user role is Super Admin. "
        "(e.g., --assign-agent agent_id_1,agent_id_2,...)"
    ),
)

After:

@click.option(
    "--assign-agent",
    default=None,
    type=list_expr,
    help=(
        "Assign specific agents to the session. "
        "This option is only applicable when the user role is Super Admin. "
        "(e.g., --assign-agent agent_id_1,agent_id_2,...)"
    ),
)

PR description

resolves lablup#5