The graph documentation missed a few semicolons

The graph documentation missed a few semicolons after an import statement, which results in errors on tutor pages. For example, see the [Graph::predecessors](http://tutor.rascal-mpl.org/Rascal/Rascal.html#/Rascal/Libraries/analysis/graphs/Graph/predecessors/predecessors.html) documentation:
```
rascal>import analysis::graphs::Graph
>>>>>>>predecessors({<1,2>, <1,3>, <2,4>, <3,4>}, 4);
        ^ Parse error here
```