Fix translations link in PR template (#451)

Author: serima

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,6 +6,6 @@ Before submitting a pull request, verify it meets all requirements in the [Contr
 
 See the [Contributing Guidelines](https://github.com/donnemartin/system-design-primer/blob/master/CONTRIBUTING.md).  Verify you've:
 
-* Tagged the [language maintainer](TRANSLATIONS.md)
+* Tagged the [language maintainer](https://github.com/donnemartin/system-design-primer/blob/master/TRANSLATIONS.md)
 * Prefixed the title with a language code
     * Example: "ja: Fix ..."