fix help message in chgrp

[m11o]

issue: #4368

This PR contains bellow contents.

• add command name in usage block in chgrp.md
• use const instead of static

chgrp --help outputs follow.

before

$ ./target/debug/chgrp --help
Change the group of each FILE to GROUP.

Usage: ./target/debug/chgrp [OPTION]... GROUP FILE...
       ./target/debug/chgrp --reference=RFILE FILE...

Arguments:
  <GROUP>
  <FILE>...

Options:
      --help               Print help information.
  -c, --changes            like verbose but report only when a change is made
  -f, --silent
      --quiet              suppress most error messages
  -v, --verbose            output a diagnostic for every file processed
      --dereference
  -h, --no-dereference     affect symbolic links instead of any referenced file (useful only on systems that can change the ownership of a symlink)
      --preserve-root      fail to operate recursively on '/'
      --no-preserve-root   do not treat '/' specially (the default)
      --reference <RFILE>  use RFILE's group rather than specifying GROUP values
  -R, --recursive          operate on files and directories recursively
  -H                       if a command line argument is a symbolic link to a directory, traverse it
  -P                       do not traverse any symbolic links (default)
  -L                       traverse every symbolic link to a directory encountered
  -V, --version            Print version

after

$ ./target/debug/chgrp --help
Change the group of each FILE to GROUP.

Usage: ./target/debug/chgrp [OPTION]... GROUP FILE...
       ./target/debug/chgrp [OPTION]... --reference=RFILE FILE...

Arguments:
  <GROUP>
  <FILE>...

Options:
      --help               Print help information.
  -c, --changes            like verbose but report only when a change is made
  -f, --silent
      --quiet              suppress most error messages
  -v, --verbose            output a diagnostic for every file processed
      --dereference
  -h, --no-dereference     affect symbolic links instead of any referenced file (useful only on systems that can change the ownership of a symlink)
      --preserve-root      fail to operate recursively on '/'
      --no-preserve-root   do not treat '/' specially (the default)
      --reference <RFILE>  use RFILE's group rather than specifying GROUP values
  -R, --recursive          operate on files and directories recursively
  -H                       if a command line argument is a symbolic link to a directory, traverse it
  -P                       do not traverse any symbolic links (default)
  -L                       traverse every symbolic link to a directory encountered
  -V, --version            Print version

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail-2/inotify-dir-recreate is no longer failing!

[cakebaker]

Thanks!