docs: borg-serve: simplify example of env in authorized_keys

[qyanu]

see #8318

so long as it can be assumed that the user has configured a POSIX compliant login shell, using a simple command [1] looks cleaner, as no export or ; are used.

[1] Section "2.9.1 Simple Commands" in volume "Shell & Utilities" of POSIX.1-2024

[qyanu]

While thinking about issue https://github.com/borgbackup/borg/issues/8318 i noticed, that the recommended authorized_keys command to set a BORG_XXX environment variable can do without the ";" and "export". so i just quickly created this pull request.

[ThomasWaldmann]

Looks good IMHO/AFAIK.

I guess we'll keep this open for a while in case somebody has objections.

[ThomasWaldmann]

Thanks!