Add roberta to doc

examples/run_ner.py

@@ -13,7 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-""" Fine-tuning the library models for named entity recognition on CoNLL-2003 (Bert). """
+""" Fine-tuning the library models for named entity recognition on CoNLL-2003 (Bert or Roberta). """
 
 from __future__ import absolute_import, division, print_function