Fixing kubectl command in walkthrough documentation

[arschles]

The existing port-forward command used the k alias. This PR:

• changes that command to be kubectl so that folks who don't have that alias set up will still be able to use it properly
• emphasizes the need to set up your NAMESPACE env var.

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• Any necessary documentation is added, such as:
  • README.md
  • The docs/ directory
  • The docs repo