Fix /docs/witness_completion.md formatting

Lines ~7-31 of the witness_completion.md doc seem to be incorrectly formatted per GFM: https://github.com/testifysec/witness/blob/602dc485b2b775e6c7cc66b11653a91fdef2aeda/docs/witness_completion.md?plain=1#L7-L31

Screenshot of rendered Preview:

There are a few ways to take care of this but this is perhaps the simplest for what was intended:

To load completions:

Bash:
```
  $ source <(witness completion bash)
  # To load completions for each session, execute once:   
  # Linux:
  $ witness completion bash > /etc/bash_completion.d/witness
  # macOS:
  $ witness completion bash > /usr/local/etc/bash_completion.d/witness
```
Zsh:
```
  # If shell completion is not already enabled in your environment,
  # you will need to enable it.  You can execute the following once:
  $ echo "autoload -U compinit; compinit" >> ~/.zshrc
  # To load completions for each session, execute once:
  $ witness completion zsh > "${fpath[1]}/_witness"
  # You will need to start a new shell for this setup to take effect.
```
fish:
```
  $ witness completion fish | source
  # To load completions for each session, execute once:
  $ witness completion fish > ~/.config/fish/completions/witness.fish
```
PowerShell:
```
  PS> witness completion powershell | Out-String | Invoke-Expression
  # To load completions for every new session, run:
  PS> witness completion powershell > witness.ps1
  # and source this file from your PowerShell profile.
```

Which renders this:

I would put up a PR after raising this, but I'm not sure this constitutes a "meaningful body of work" haha.

in-toto / witness

Fix /docs/witness_completion.md formatting #291