subset.ipynb: fix output change for new Jupyter env (#275)

Related to Jupyter env
Ouranosinc/PAVICS-e2e-workflow-tests#115

To fix this error:
```
  ___________ finch-master/docs/source/notebooks/subset.ipynb::Cell 16 ___________
  Notebook cell execution failed
  Cell 16: Cell outputs differ

  Input:
  all_shapes.plot()

  Traceback:
   mismatch 'text/plain'

   assert reference_output == test_output failed:

    '<AxesSubplot:>' == '<Axes: >'
    - <Axes: >
    + <AxesSubplot:>
```

Makefile

@@ -126,7 +126,7 @@ notebook-sanitizer:
 .PHONY: test-notebooks
 test-notebooks: notebook-sanitizer
 	@echo "Running notebook-based tests"
-	@bash -c "env WPS_URL=$(WPS_URL) pytest --nbval --verbose $(CURDIR)/docs/source/notebooks/ --sanitize-with $(CURDIR)/docs/source/output-sanitize.cfg --ignore $(CURDIR)/docs/source/notebooks/.ipynb_checkpoints"
+	@bash -c "env WPS_URL=$(WPS_URL) pytest --nbval --verbose $(CURDIR)/docs/source/notebooks/ --nbval-sanitize-with $(CURDIR)/docs/source/output-sanitize.cfg --ignore $(CURDIR)/docs/source/notebooks/.ipynb_checkpoints"
 
 .PHONY: lint
 lint: