Update GNN example readme (#283)

`chardet` package now required by stellargraph
Also, switching to mamba for speed and conda seemed to struggle solving env.
Not sure if we should start pinning these?

Authors:
  - Pete MacKinnon (https://github.com/pdmack)

Approvers:
  - Christopher Harris (https://github.com/cwharris)

URL: #283

examples/gnn_fraud_detection_pipeline/README.md

@@ -21,7 +21,7 @@ limitations under the License.
 Prior to running the gnn fruad detection pipeline, additional requirements must be installed in to your conda environment. A supplemental requirements file has been provided in this example directory.
 
 ```bash
-conda install -c rapidsai -c nvidia -c stellargraph -c conda-forge cuml stellargraph tensorflow
+mamba install -c rapidsai -c nvidia -c stellargraph -c conda-forge chardet cuml stellargraph tensorflow
 ```
 
 ## Running