fix: Remove typo in Alternate signing flow

[elfotografo007]

Signed-off-by: Andrés Torres [email protected]

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Fixes #
Release Notes:

Types of changes:

• Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of the changes being introduced by the pull request:
Remove typo in Alternate signing flow section of README. Running the current command was returning:

tuff add-signatures --signatures=root.json.sigs root.json
--signatures must not have an argument

With the suggestion, the command works as expected:

tuf add-signatures --signatures root.json.sigs root.json
tuf: added 1 new signature(s)

Please verify and check that the pull request fulfills the following requirements:

• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[joshuagl]

LGTM, thank you for taking the time to submit.