Fix path to TestShop.AppHost in contributing.md

[mfcollins3]

Description

The instructions in contributing.md for running the TestShop AppHost project had an incorrect path. I corrected the documentation with the correct path so that the instructions are accurate.

Fixes #6479

Checklist

• Is this feature complete?
  • [x] Yes. Ready to ship.
  • [ ] No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • [ ] Yes
  • [x] No
• Did you add public API?
  • [ ] Yes
  • If yes, did you have an API Review for it?
    • [ ] Yes
    • [ ] No
  • Did you add <remarks /> and <code /> elements on your triple slash comments?
    • [ ] Yes
    • [ ] No
  • [x] No
• Does the change make any security assumptions or guarantees?
  • [ ] Yes
  • If yes, have you done a threat model and had a security review?
    • [ ] Yes
    • [ ] No
  • [x] No

• Does the change require an update in our Aspire docs?

  • [ ] Yes
  • Link to aspire-docs issue:
  • [x] No

  Microsoft Reviewers: Open in CodeFlow

[radical]

Thank you for the PR!