patch 9.1.0845: vimtutor shell script can be improved

patch 9.1.0845: vimtutor shell script can be improved

Problem:  vimtutor shell script can be improved
Solution: further improve the vimtutor shell script
          (Aliaksei Budavei)

- Rewrite the script usage note.
- Reconcile the usage help output with the manual page entry
  that describes the implementation in that:
  * a language code argument can be used alone or with its
    option key, e.g. "vimtutor -l nl" or "vimtutor nl";
  * a chapter number argument cannot be used without its
    option key, e.g. "vimtutor -c 2".
- Accept only chapters 1 or 2 as valid chapter arguments.
- Double-quote instances of shell parameter expansion where
  neither pathname expansion nor field splitting is desired.
- Prefer "$(foo)" to "`foo`" for command substitution.
- Follow a single indentation style (see the modeline).

closes: #15992

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>