Fix: add missing value in 'board list' table

[cmaglie]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?
Fix a missing value in the board list table output.

What is the current behavior?

$ arduino-cli board list
Porta        Protocollo Tipo    Nome scheda FQBN Core
/dev/ttyUSB0 serial     Unknown                 

What is the new behavior?

$ arduino-cli board list
Porta        Protocollo Tipo              Nome scheda FQBN Core
/dev/ttyUSB0 serial     Serial Port (USB) Unknown

Does this PR introduce a breaking change, and is
titled accordingly?
No