Fix a typo in _msfvenom

[zeroSteiner]

Fixes #16838

This fixes a typo in the ZSH completion file for _msfvenom that would cause an error when completing the --arch argument.

Verification

• [ ] Install zsh
• [ ] Install the _msfvenom completion file
• [ ] Initialize it so the completions are loaded
• [ ] Type ./msfvenom --arch and hit tab, you should see architectures and not an error