Fix .env file path in integration_test README.md (#14028)

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### Description
Hello, 

The [integration_test
README](https://github.com/langchain-ai/langchain/tree/master/libs/langchain/tests)
was indicating incorrect paths for the `.env.example` and `.env` files.

`tests/.env.example` ->`tests/integration_tests/.env.example`

While it’s a minor error, it could **potentially lead to confusion** for
the document’s readers, so I’ve made the necessary corrections.

Thank you! ☺️

### Related Issue
- #2806

libs/langchain/tests/README.md

@@ -77,8 +77,8 @@ For environments that requires more involving preparation, look for `*.sh`. For
 
 ### Prepare environment variables for local testing:
 
-- copy `tests/.env.example` to `tests/.env`
-- set variables in `tests/.env` file, e.g `OPENAI_API_KEY`
+- copy `tests/integration_tests/.env.example` to `tests/integration_tests/.env`
+- set variables in `tests/integration_tests/.env` file, e.g `OPENAI_API_KEY`
 
 Additionally, it's important to note that some integration tests may require certain
 environment variables to be set, such as `OPENAI_API_KEY`. Be sure to set any required