Update CONTRIBUTING.md

Fixed link to Issue Template

.github/CONTRIBUTING.md

@@ -4,7 +4,7 @@ If you have questions about how to install or use Nextcloud, please direct these
 
 ### Short version
 
- * The [**issue template can be found here**][template] but be aware of the different repositories! See list below. Please always use the issue template when reporting issues.
+ * The [**issue template can be found here**][ISSUE_TEMPLATE] but be aware of the different repositories! See list below. Please always use the issue template when reporting issues.
 
 ### Guidelines
 * Please search the existing issues first, it's likely that your issue was already reported or even fixed.