Update README.md of wdio-allure-reporter

[goatsy]

afterTest Hook has changed in WebdriverIO 6. In WebdriverIO version 6 test.error does not exist anymore

Proposed changes

Types of changes

• Bugfix (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)
• Documentation update

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Further comments

Reviewers: @webdriverio/project-committers

[christian-bromann]

👍