docs: fix `download & expand` instruction ( `tar -xzf` is Linux-specific )

[kanarus]

In current README:

• Example commands to download & expand MuJoCo 3.3.2:

  wget https://github.com/google-deepmind/mujoco/releases/download/3.3.2/mujoco-3.3.2-＜YOUR OPTION＞
  tar -xzf mujoco-3.3.2-＜YOUR OPTION＞

  Replace ＜YOUR OPTION＞ depending on your system.

is instructed in the Note & Tips section, but this is Linux-specific:

• for Mac mujoco-<version>-macos-universal2.dmg is provided
• for Windows mujoco-<version>-windows-x86_64.zip is provided

in the Release page.