doc: update directories to directory in corepack --help

[lvqq]

As option --install-directory is a single specified directory, so the command description is better to be directory

━━━ General commands ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  corepack disable [--install-directory #0] ...
    Remove the Corepack shims from the install directory

  corepack enable [--install-directory #0] ...
    Add the Corepack shims to the install directories

  corepack hydrate [--activate] <fileName>
    Import a package manager into the cache

  corepack prepare [--activate] [--all] [--json] [-o,--output] ...
    Generate a package manager archive

You can also print more details about any of these commands by calling them with
the `-h,--help` flag right after the command name.