docs: fix virtual environment instruction typo

[leno23]

Summary

• Correct Active the virtual environment to Activate the virtual environment in the setup steps.

Fixes #17

Test plan

• static validation: Active the virtual environment appeared 1 time(s) before replacement.
• static validation: Active the virtual environment absent and Activate the virtual environment present after patch.
• Not run: runtime test suite, because this is a documentation-only fix.

[tim-schilling]

Thanks @leno23!