Fix command syntax in README for building firmware

[psiddh]

Summary

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 1f37e98 with merge base a0ba28e ():

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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[Copilot]

Pull request overview

This pull request fixes the command syntax in the README for building firmware on Raspberry Pi Pico2 by adding the ./ prefix to the shell script invocation.

Changes:

• Corrects the shell script invocation from build_firmware_pico.sh to ./build_firmware_pico.sh in the build firmware instructions

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