Update Internet#username separator param to match with the example

[AlexandreL0pes]

Motivation / Background

This PR is a minor documentation fix. Using this param separators: ['-'] on Faker::Internet.username method will return a username with - and not _ as the example. I have updated the param to match with the current example.

Example:

irb(main):006:0> Faker::Internet.username(specifier: 20, separators: ['-'])
=> "lauren-connlauren-conn"
irb(main):007:0> Faker::Internet.username(specifier: 20, separators: ['_'])
=> "toccara_nikolaustoccara_nikolaus"

Additional information

This change is small and simple but would help to maintain the integrity of the doc.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• No tests required as this is a doc update.
• Tests and Rubocop are passing before submitting your proposed changes.

[thdaraujo]

👍