Fix zsh completion

[umbynos]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?

Bug fix

• What is the current behavior?

Before this PR the zsh completion was not working correctly: when you tried to press key after pressing "-" you would expect to receive only the flag name and not all the description alongside the name of the flag. This PR fixes that problem.
Before:

• What is the new behavior?

After:

• Does this PR introduce a breaking change?

no

• Other information:

To help the review:

1. Checkout this branch
2. task build
3. ./arduino-cli completion zsh > _arduino-cli
4. source _arduino-cli
5. rm -f ~/.zcompdump; compinit
6. try out the completion

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See how to contribute

[ubidefeo]

LGTM