docs: clarify PMC PowerShell command wording

[EduardF1]

Summary

Fixes #4974

Changes

• clarify that the install and update snippets are PowerShell commands run inside Package Manager Console
• keep the existing snippets unchanged while removing the labeling mismatch noted in the issue

Testing

• ran git diff --check
• reviewed the rendered markdown change in entity-framework/core/cli/powershell.md