Edit sign up announcement banner

[mesellings]

Description

Correct the "Sign-Up" text in the docs site announcement banner to "Sign up".

Note: As this is in the main docusaurus config file, it is version-agnostic and is changed for the whole site, regardless of which docs version you select.

Linked Issue: #4093

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[akeller]

🔍 Thank you!

[github-actions]

🧹 Preview environment for this PR has been torn down.