Update CONTRIBUTING.md (#4486)

Typo in the filename.

Author: zerocukor287

.github/CONTRIBUTING.md

@@ -20,7 +20,7 @@ Clearly describe the issue:
 - If you propose a change or addition, try to give an **example** how the improved code could look like or how to use it.
 - If you found a compilation error, please tell us which **compiler** (version and operating system) you used and paste the (relevant part of) the error messages to the ticket.
 
-Please stick to the provided issue template ([bug report](https://github.com/nlohmann/json/blob/develop/.github/ISSUE_TEMPLATE/bug.yml) if possible. For questions, feature or support requests, please [open a discussion](https://github.com/nlohmann/json/discussions/new).
+Please stick to the provided issue template ([bug report](https://github.com/nlohmann/json/blob/develop/.github/ISSUE_TEMPLATE/bug.yaml) if possible. For questions, feature or support requests, please [open a discussion](https://github.com/nlohmann/json/discussions/new).
 
 ## Files to change