Fix broken deployment link in Flask REST API tutorial

[Rachel0619]

Fixes #3044

Description

Fixed the broken example deployment link in the "Deploying PyTorch in Python via a REST API with Flask" tutorial. The previous link was no longer accessible, so I updated it with a working Heroku template URL according to suggestions from @harshaljanjani

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes Example deployment link broken in "Deploying PyTorch in Python via a REST API with Flask" #3044")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

cc @svekars @sekyondaMeta @AlannaBurke

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3385

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