fix broken link in pr template (#7880) [no ci]

* fix broken link in pr template

* Update pull_request_template.md [no ci]

---------

Co-authored-by: Brian <mofosyne@gmail.com>

.github/pull_request_template.md

@@ -2,4 +2,4 @@
     - [ ] Review Complexity : Low
     - [ ] Review Complexity : Medium
     - [ ] Review Complexity : High
-- [ ] I have read the [contributing guidelines](CONTRIBUTING.md)
+- [ ] I have read the [contributing guidelines](https://github.com/ggerganov/llama.cpp/blob/master/CONTRIBUTING.md)