Fix new surface provider camerax PR url

[ThibaultBee]

The current URL https://github.com/flutter/flutter/packages/pull/7044 does not exist, it should be https://github.com/flutter/packages/pull/7044 instead

Description of what this PR is changing or adding, and why:

In page about the new surface provider:
The URL of this camera_android_camerax PR https://github.com/flutter/flutter/packages/pull/7044 does not exist, it should be flutter/packages#7044 instead.

Issues fixed by this PR (if any):

None

PRs or commits this PR depends on (if any):

None

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[sfshaza2]

/gcbrun

[sfshaza2]

lgtm

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 242be57):

https://flutter-docs-prod--pr11227-patch-1-pnxs5grs.web.app