docs: correct spelling of Spanner in README.rst (#179)

Co-authored-by: Averi Kitsch <akitsch@google.com>

Author: ghadlich

README.rst

@@ -73,7 +73,7 @@ Use a vector store to store embedded data and perform vector search.
 
 .. code-block:: python
 
- from langchain_google_sapnner import SpannerVectorstore
+ from langchain_google_spanner import SpannerVectorstore
  from langchain.embeddings import VertexAIEmbeddings
 
  embeddings_service = VertexAIEmbeddings(model_name="textembedding-gecko@003")