Fix type error in GraphMetrics.java

How to reproduce:

    git clone git@github.com:TU-Berlin-DIMA/flink-graph.git
    cd flink-graph
    mvn clean install

Output:

    ...
    [ERROR] ~/flink-graph/src/main/java/flink/graphs/example/GraphMetrics.java:[67,64] method map in class org.apache.flink.api.java.DataSet<T> cannot be applied to given types;
    [ERROR] required: org.apache.flink.api.common.functions.MapFunction<org.apache.flink.api.java.tuple.Tuple,R>
    [ERROR] found: flink.graphs.example.GraphMetrics.AvgNodeDegreeMapper
    [ERROR] reason: no instance(s) of type variable(s) R exist so that argument type flink.graphs.example.GraphMetrics.AvgNodeDegreeMapper conforms to formal parameter type org.apache.flink.api.common.functions.MapFunction<org.apache.flink.api.java.tuple.Tuple,R>
    ...