Fix typo

Author: szarnyasg

src/main/java/ldbc/snb/datagen/generator/generators/PersonActivityGenerator.java

@@ -120,7 +120,7 @@ private GenWall<Triplet<Post, Stream<Like>, Stream<Pair<Comment, Stream<Like>>>>
         // Generate wall
         Forum wall = forumGenerator.createWall(randomFarm, startForumId++, person, blockId);
 
-        // Could be null is moderator can't be added
+        // Could be null as moderator can't be added
         if (wall == null)
             return new GenWall<>(Stream.empty());