docs: Add embedding_model attribute when creating TestsetGenerator object (#1606)

- edit the document because embedding_model attribute has been added to
the TestsetGenerator object.
- #1562

Author: Youngrok123

docs/getstarted/rag_testset_generation.md

@@ -39,7 +39,7 @@ Now we will run the test generation using the loaded documents and the LLM setup
 ```python
 from ragas.testset import TestsetGenerator
 
-generator = TestsetGenerator(llm=generator_llm)
+generator = TestsetGenerator(llm=generator_llm, embedding_model=generator_embeddings)
 dataset = generator.generate_with_langchain_docs(docs, testset_size=10)
 ```