Fix grammar in AddX method responsibilities docs

[Brett Cannon (brettcannon)]

Description

The docs said, "executes" but then said "validate inputs", which didn't read smoothly to me. Saying the method "will" "validate inputs" seems like a better fit.

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • 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
  • 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
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire-docs issue (consider using one of the following templates):
      • New (or update) doc-idea template
      • New breaking-change template
      • New diagnostic template
  • No

[Copilot]

Pull Request Overview

A minor grammar improvement to the documentation for AddX method responsibilities, changing "executes" to "will" for better readability and flow when describing what the method will do.

---

_{You can also share your feedback on Copilot code review for a chance to win a $100 gift card. Take the survey.}