Update Prater network to Goerli

[rolandtyler]

Pull request checklist

Use the following template to make sure your PR fits the Web3Signer documentation standard.

Before creating the PR

Make sure that:

• You've read the contribution guidelines.
• You've previewed your changes locally.

Describe the change

Update Prater network to Goerli

Issue fixed

Fixes #132

Impacted parts

For content changes:

• Documentation content
• Documentation page organization

For tool changes:

• CircleCI workflow
• Build and QA tools (for example, lint or vale)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• Read the Docs configuration
• GitHub integration

After creating your PR and tests have finished

Make sure that:

• You've fixed any issues raised by the tests.
• You've previewed your changes on Read the Docs
  and added a preview link.

Preview

[alexandratran]

LGTM.