Fixed a copy paste errors and filtered out all edges with form (x,x)

Before, there were edges in this form (x,x) included but they shouldn't be included.

documents/musterloesung-db-2012-09-24/d2c2.sql

@@ -4,9 +4,10 @@ FROM (
 ) f1
 JOIN ON
 (
-    SELECT personni FROM FriendshipSymmetric WHERE person1 = <id>
+    SELECT person2 FROM FriendshipSymmetric WHERE person1 = <id>
 ) f2
 EXCEPT
 (
     SELECT * FROM FriendshipSymmetric
 )
+WHERE f1.person2 != f2.person2