Update Testing.md

[tsubasaxZZZ]

This pull request includes a change to the Documentation/Testing.md file.
The change adds a note to the documentation to guide users on the correct way to access the Package in their browser, specifically emphasizing not to put a slash at the end of the Package parameter.

• Documentation/Testing.md: Added a note to guide users on the correct way to access the Package in their browser, emphasizing not to put a slash at the end of the Package parameter.

PR Checklist

• Explain your changes, so people looking at the PR know what and why, the code changes are the how.
• Validate your changes using one or more of the testing methods.

If adding or updating templates:

• post a screenshot of templates and/or gallery changes
• ensure your template has a corresponding gallery entry in the gallery folder
• If you are adding a new template, add your team and template/gallery file(s) to the CODEOWNERS file. CODEOWNERS entries should be teams, not individuals
• ensure all steps have meaningful names
• ensure all parameters and grid columns have display names set so they can be localized
• ensure that parameters id values are unique or they will fail PR validation (parameter ids are used for localization)
• ensure that steps names are unique or they will fail PR validation (step names are used for localization)
• grep /subscription/ and ensure that your parameters don't have any hardcoded resourceIds or they will fail PR validation
• remove fallbackResourceIds and fromTemplateId fields from your template workbook or they will fail PR validation