Update typo in link documentation

[noe-sautel]

Pull Request Description

Summary

This pull request updates the Heroku deployment URL in the documentation to resolve the issue with missing app.json in the provided repository URL.

Changes

• Updated the Heroku deployment URL:
  Changed the deployment link from:
  https://heroku.com/deploy?template=https://github.com/fbsamples/graph-api-webhooks-samples/
  to:
  https://heroku.com/deploy?template=https://github.com/fbsamples/graph-api-webhooks-samples

Context

• This modification is necessary because users attempting to deploy the sample app via Heroku were encountering the error message:
  "No app.json located in the repo URL provided."
• The issue is documented in Issue #34, where user @erickwinz30 reported the problem when trying to deploy the app on Heroku.

Instructions

1. Refer to Facebook's Webhooks sample app documentation to understand how to use this application.

2. To deploy the sample app on Heroku, use the updated button below:

   

Instagram Subscription API

• If you're working with Instagram, you can register an Instagram API client as part of the setup.

This update should resolve the deployment issue by ensuring the correct Heroku template URL is used for proper app configuration.

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