fix: Fix link to CONTRIBUTING.md in PR template (#4808)

### Description
The link to CONTRIBUTING.md in the PR template doesn't work (see below),
updating it.

#### Issues
<!--
* resolves: #1234
* resolves: LIN-1234
-->

#### Reminders
- Please add tests to validate your changes, and lint your code using
`tox -e linters`.
- Add GH Issue ID _&_ Linear ID (if applicable)
- PR title should use [conventional
commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors: [CONTRIBUTING.md](../CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

Author: sentrivana

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,4 +11,4 @@
 - Please add tests to validate your changes, and lint your code using `tox -e linters`.
 - Add GH Issue ID _&_ Linear ID (if applicable)
 - PR title should use [conventional commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type) style (`feat:`, `fix:`, `ref:`, `meta:`)
-- For external contributors: [CONTRIBUTING.md](../CONTRIBUTING.md), [Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord community](https://discord.gg/Ww9hbqr)
+- For external contributors: [CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md), [Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord community](https://discord.gg/Ww9hbqr)