fix: update mo2 'getprefix' arg to match help docs

[koppchen]

Small fix to change the mo2 getpfx option to mo2 getprefix, which matches what's in the howto/help docs.

I chose this route over updating the docs since it matched the vortex getprefix|gp args, but feel free to change it.

Testing (before)

Show that the howto/help docs list mo2 getprefix|gp options. (this PR doesn't change this)

$ steamtinkerlaunch --help | grep -A 3 " mo2"
    mo2                              Mod Organizer 2
         create-instance|ci <id>     create MO2 instance for <id>
         download|d                  download latest MO2 release
         getprefix|gp                Output path to MO2 prefix

Using steamtinkerlaunch mo2 getprefix displays the howto/help docs

$ steamtinkerlaunch mo2 gp
/home/user/.config/steamtinkerlaunch/mo2/compatdata/pfx

$ steamtinkerlaunch mo2 getprefix | head -n 4
=========================
SteamTinkerLaunch v14.0.20240423-1
=========================
Usage: steamtinkerlaunch [options]...

Testing (after)

Using steamtinkerlaunch mo2 getprefix matches the mo2 gp output.

$ steamtinkerlaunch mo2 gp
/home/user/.config/steamtinkerlaunch/mo2/compatdata/pfx

$ steamtinkerlaunch mo2 getprefix
/home/user/.config/steamtinkerlaunch/mo2/compatdata/pfx

[sonic2kk]

This looks like a silly mistake on my part. Thanks for fixing it! Will merge

[sonic2kk]

oops, meant to bump the version. Will push another commit to do it.

This fix won't be noted on the changelog because it fixes something not yet in a release, I hope that's okay. Thank you very much for the contribution nonetheless!