doc: Dead link issue in 'contribute/proposals_and_rfcs'

**Describe the bug**

Dead link:
https://github.com/zephyrproject-rtos/zephyr/blob/main/.github/ISSUE_TEMPLATE/003_rfc-proposal.md

what i think the target should be:
https://github.com/zephyrproject-rtos/zephyr/blob/main/.github/ISSUE_TEMPLATE/003_rfc-proposal.yml



**Environment**

* Page: `contribute/proposals_and_rfcs`
* Version: main
* SHA-1: c7ef2e9d5c3881545c932f939a19b3a8fd168095
