docs: update contributing guidelines link (#17)

Author: sibiraj-s

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,7 +4,7 @@ Describe the goal of this PR. Mention any related Issue numbers.
 
 ### Requirements (place an `x` in each `[ ]`)
 
-* [ ] I've read and understood the [Contributing Guidelines](https://github.com/slackhq/csp-html-webpack-plugin/blob/master/.github/contributing.md) and have done my best effort to follow them.
+* [ ] I've read and understood the [Contributing Guidelines](https://github.com/slackhq/csp-html-webpack-plugin/blob/master/.github/CONTRIBUTING.md) and have done my best effort to follow them.
 * [ ] I've read and agree to the [Code of Conduct](https://slackhq.github.io/code-of-conduct).
 * [ ] I've written tests to cover the new code and functionality included in this PR.
 * [ ] I've read, agree to, and signed the [Contributor License Agreement (CLA)](https://cla-assistant.io/slackhq/csp-html-webpack-plugin).