fix: wrong description on pixi project help

[notPlancha]

channel main docstring and platform main docstring are the same, same thing with description and version. Doubt it's intentional, easy change.

Before:

Commands:
  channel      Commands to manage project channels
  description  Commands to manage project description
  platform     Commands to manage project channels
  version      Commands to manage project description
  help         Print this message or the help of the given subcommand(s)

After:

Commands:
  channel      Commands to manage project channels
  description  Commands to manage project description
  platform     Commands to manage project platforms
  version      Commands to manage project version
  help         Print this message or the help of the given subcommand(s)