shell: shell_help: terminal offset of subcommands' help text isn't aligned

[ycsin]

Describe the bug
The help message of a shell subcommand isn't aligned.

To Reproduce
Steps to reproduce the behavior:

1. Boot something with devmem enabled on qemu
2. Type devmem -h in shell
3. See error

Expected behavior
Texts should be aligned

Impact
Text not aligned

Logs and console output

uart:~$ devmem -h
devmem - Read/write physical memory
         Usage:
         Read memory at address with optional width:
         devmem address [width]
         Write memory at address with mandatory width and value:
         devmem address <width> <value>
Subcommands:
  dump  : Usage:
         devmem dump -a <address> -s <size> [-w <width>]
        ~~~
         ^ not aligned

  load  : Usage:
         devmem load [options] [address]
        ~~~
         ^ not aligned
Options:
-e      little-endian parse

Environment (please complete the following information):

• OS: Linux
• Toolchain Zephyr SDK 0.16.8
• Commit SHA or Version used: v3.7-branch

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