Fix incorrect verbose flag in troubleshoot section

[Keqy]

Change -verbose to --verbose in flutter doctor command to match actual CLI behavior

Description of what this PR is changing or adding, and why:
Fixed incorrect flag in the troubleshoot documentation.
The documentation showed "Running flutter doctor –verbose gives..."
but the correct flag is flutter doctor --verbose. This was verified by running flutter doctor --help:
-v, --verbose Noisy logging, including all shell commands executed.
flutter doctor -verbose gives an error:

Could not find an option with short name "-e".
Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.

Issues fixed by this PR (if any):
None
PRs or commits this PR depends on (if any):
None

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

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[Keqy]

I have signed the CLA.