[documentation] DataFrame column name is wrong

[pfrischmann]

In upsert_from_dataframe, the correct DataFrame column name is values, and NOT vector

pinecone-python-client/pinecone/data/index.py

Line 230 in d9df375

df: A pandas dataframe with the following columns: id, vector, sparse_values, and metadata.

[anawishnoff]

Hey @jseldess, would you mind making this quick fix?