Update README.md: Added ubuntu command that is required before running apt install

[Jeanclaudeaoun]

Description

Added ubuntu command that is required before running apt install to get package lists

Motivation and Context

To prevent the error: E: Unable to locate package python3-virtualenv

How Has This Been Tested?

while installing on ubuntu 20

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[CLAassistant]

All committers have signed the CLA.