removing SimpleMongoConverter references from javadocs

In commit 2832b524d32107e5b24c33cf4d08035e1bcec6c1 MappingMongoConverter was made default instead of SimpleMongoConverter. Also SimpleMongoConverter was completely removed between 1.0.0.M3 and 1.0.0.M4 releases. This is an update for javadocs, that still reference SimpleMongoConverter as the default MongoConverter.