Tags a doc test as ignored because it relies on an optional feature

lib/src/model/dataset.rs

@@ -123,7 +123,7 @@ pub trait NamedGraph: Graph {
 /// ```
 ///
 /// The implementation backed by RocksDB could be built using `RocksDbDataset::open` and works just like its in-memory equivalent:
-/// ```
+/// ```ignore
 /// use rudf::store::RocksDbDataset;
 /// let dataset = RocksDbDataset::open("foo.db");
 /// ```