docs: QdrantDocumentStore initialization example doesn't work

[TaMaN2031A]

Describe the bug
When I run the example here, the code doesn't work because it tries to insert a vector with a dimension different from the default dimension the document store was created with.

Error message
ValueError: could not broadcast input array from shape (5,) into shape (768,)

Expected behavior
The code should work normally.

Additional context
The vector size should be 768.

To Reproduce
Run the code.

FAQ Check

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System:

• OS: wsl2 on windows 11
• Haystack version (commit or version number): 2.20.0
• DocumentStore: QdrantDocumentStore