fix(steamcmd): fix beta branch support

[Frisasky]

Description

According to steamcmd document, the proper way to designate a branch in steamcmd parameter is "-beta branch" rather than simply put “${branch}" variable in parameter

Fixes #2531

Type of change

• Bug fix (change which fixes an issue).
• New feature (change which adds functionality).
• New Server (new server added).
• Refactor (restructures existing code).
• Comment update (typo, spelling, explanation, examples, etc).

Checklist

PR will not be merged until all steps are complete.

• This pull request links to an issue.
• This pull request uses the develop branch as its base.
• This code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have checked that this code is commented where required.
• I have provided a detailed enough description of this PR.
• I have checked If documentation needs updating.

Documentation

If documentation does need updating either update it by creating a PR (preferred) or request a documentation update.

• User docs: https://github.com/GameServerManagers/LinuxGSM-Docs
• Dev docs: https://github.com/GameServerManagers/LinuxGSM-Dev-Docs

Thank you for your Pull Request!

[dgibbs64]

Docs need updating for this https://docs.linuxgsm.com/steamcmd/branch

Branch

Allows the selection of specific builds of a game server in SteamCMD