docs: shorten and correct the issue template link

Author: Sam Harrison

CONTRIBUTING.md

@@ -52,7 +52,7 @@ Before you decide to create a new issue, please try the following:
 
 ### Please use our Bug Report Template
 
-To make the process easier, we've included a [sample bug report template](https://github.com/sendgrid/php-http-client/ISSUE_TEMPLATE.md) (borrowed from [Ghost](https://github.com/TryGhost/Ghost/)). The template uses [GitHub flavored markdown](https://help.github.com/articles/github-flavored-markdown/) for formatting.
+In order to make the process easier, we've included a [sample bug report template](ISSUE_TEMPLATE.md).
 
 <a name="improvements-to-the-codebase"></a>
 ## Improvements to the Codebase