Remove link formatting for release contributors.

Use plain GitHub user handles in release to use GitHub's contributor feature.

Closes #51

src/main/java/org/springframework/data/release/issues/github/ChangelogGenerator.java

@@ -138,7 +138,7 @@ public class ChangelogGenerator {
 	}
 
 	private String formatContributors(GitHubUser c) {
-		return String.format("- [@%s](%s)%n", c.getName(), c.getUrl());
+		return String.format("- @%s%n", c.getName());
 	}
 
 }