docs: Updated quickstart docs to import FeatureService

[franciscojavierarceo]

The quickstart is missing an import.

What this PR does / why we need it:

Just updates the quickstart notebook.

[felixwang9817]

/lgtm

@franciscojavierarceo thanks for the fix!

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: felixwang9817, franciscojavierarceo

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[codecov-commenter]

Codecov Report

Merging #2987 (13219d5) into master (2ef71fc) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2987      +/-   ##
==========================================
- Coverage   77.89%   77.85%   -0.05%     
==========================================
  Files         193      193              
  Lines       16589    16589              
==========================================
- Hits        12922    12915       -7     
- Misses       3667     3674       +7     

Flag	Coverage Δ
integrationtests	68.03% <ø> (-0.16%)	⬇️
unittests	58.80% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/tests/utils/basic_read_write_test.py	93.54% <0.00%> (-6.46%)	⬇️
.../integration/online_store/test_online_retrieval.py	96.84% <0.00%> (-3.16%)	⬇️
sdk/python/feast/infra/online_stores/datastore.py	87.96% <0.00%> (-1.39%)	⬇️
...east/infra/materialization/lambda/lambda_engine.py	93.75% <0.00%> (+1.04%)	⬆️

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