[skip changelog] Fix burn-bootloader command example

[silvanocerza]

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)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fixes a command example.

• What is the current behavior?

arduino-cli burn-bootloader --help prints this as an example:

Examples:
  arduino-cli burn-bootloader -b arduino:avr:uno -P atmel-ice

The programmer ID atmel-ice doesn't exist.

• What is the new behavior?

arduino-cli burn-bootloader --help now prints the correct programmer ID in the example:

Examples:
  arduino-cli burn-bootloader -b arduino:avr:uno -P atmel_ice

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Closes #1323.

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