Fix issue #686 - Users with on char username are not linked

app/src/main/java/app/fedilab/android/helper/Helper.java

@@ -317,7 +317,7 @@ public class Helper {
 
     public static final Pattern hashtagPattern = Pattern.compile("(#[\\w_A-zÀ-ÿ]+)");
     public static final Pattern groupPattern = Pattern.compile("(![\\w_]+)");
-    public static final Pattern mentionPattern = Pattern.compile("(@[\\w_.-]+[\\w])");
+    public static final Pattern mentionPattern = Pattern.compile("(@[\\w_.-]?[\\w]+)");
     public static final Pattern mentionLongPattern = Pattern.compile("(@[\\w_.-]+@[a-zA-Z0-9][a-zA-Z0-9.-]{1,61}[a-zA-Z0-9](?:\\.[a-zA-Z]{2,})+)");
 
     public static final Pattern twitterPattern = Pattern.compile("((@[\\w]+)@twitter\\.com)");