Fix broken main.tf link in tasks-docker template README (#331)

Fixes the broken main.tf link reported in the tasks-docker template
README.

The relative link `./main.tf` was returning a 404 error on the registry
website. This PR replaces it with the correct GitHub URL pointing to the
main.tf file.

**Changes:**
- Updated the main.tf link in
`/registry/coder-labs/templates/tasks-docker/README.md`
- Changed from `[main.tf](./main.tf)` to
`[main.tf](https://github.com/coder/registry/blob/main/registry/coder-labs/templates/tasks-docker/main.tf)`

Tested: Verified the new link works correctly.

---------

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

Author: blink-so[bot]

registry/coder-labs/templates/tasks-docker/README.md

@@ -63,7 +63,7 @@ Visit this URL for your Coder deployment:
 https://coder.example.com/templates/new?exampleId=scratch
 ```
 
-After creating the template, paste the contents from [main.tf](./main.tf) into the template editor and save.
+After creating the template, paste the contents from [main.tf](https://github.com/coder/registry/blob/main/registry/coder-labs/templates/tasks-docker/main.tf) into the template editor and save.
 
 Alternatively, you can use the Coder CLI to [push the template](https://coder.com/docs/reference/cli/templates_push)