Updated README example to load demo dataset

[christopherbunn]

This change adds a code snippet that loads the breast cancer demo dataset before the AutoML step.

Fixes #1084

[codecov]

Codecov Report

Merging #1151 into main will increase coverage by 0.00%.
The diff coverage is 99.95%.

@@            Coverage Diff            @@
##             main    #1151     +/-   ##
=========================================
  Coverage   99.91%   99.91%             
=========================================
  Files         183      195     +12     
  Lines       10143    11489   +1346     
=========================================
+ Hits        10134    11479   +1345     
- Misses          9       10      +1     

Impacted Files	Coverage Δ
evalml/demos/breast_cancer.py	100.00% <ø> (ø)
..._understanding/prediction_explanations/__init__.py	100.00% <ø> (ø)
...alml/objectives/binary_classification_objective.py	100.00% <ø> (ø)
evalml/pipelines/__init__.py	100.00% <ø> (ø)
evalml/pipelines/components/__init__.py	100.00% <ø> (ø)
evalml/pipelines/components/estimators/__init__.py	100.00% <ø> (ø)
...elines/components/transformers/imputers/imputer.py	100.00% <ø> (ø)
evalml/utils/gen_utils.py	98.94% <94.73%> (-1.06%)	⬇️
evalml/__init__.py	100.00% <100.00%> (ø)
evalml/automl/automl_search.py	99.58% <100.00%> (+0.02%)	⬆️
... and 88 more

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[freddyaboulton]

@christopherbunn Looks good to me!