Fix docs: query index

[MarouaneMaatouk]

Description

In this PR, there's a fix for documentation about querying indices:

index = VectorStoreIndex.from_documents(documents)
index.query(question)

which should be:

index = VectorStoreIndex.from_documents(documents)
query_engine = index.as_query_engine()
query_engine.query(question)

Fixes #874

Type of Change

Please delete options that are not relevant.

• [ ] New Loader/Tool
• [x] Bug fix / Smaller change
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [ ] Added new unit/integration tests
• [ ] Added new notebook (that tests end-to-end)
• [x] I stared at the code and made sure it makes sense

Suggested Checklist:

• [ ] I have added a library.json file if a new loader/tool was added
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I ran make format; make lint to appease the lint gods

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