Fix missing reference to `CONTRIBUTE.md` in PR template

## Description

The PR template [references](https://github.com/getodk/pyodk/blob/master/.github/PULL_REQUEST_TEMPLATE.md?plain=1#L7) a [CONTRIBUTING.md](https://github.com/getodk/pyodk/blob/master/CONTRIBUTING.md) file that does not appear to exist anymore.

## Proposal

Replace reference:

```
https://github.com/getodk/pyodk/blob/master/CONTRIBUTING.md
```

with

```
https://github.com/getodk/pyodk/blob/master/README.md#contribute
```

Open .github/PULL_REQUEST_TEMPLATE.md and inspect the reference on the mentioned line. Compare it with the README.md #contribute section, then replace the outdated CONTRIBUTING.md URL with the README link. Done means the PR template points to the existing contribution guidance.

Written by the indexing model from the issue text.