Update README.md (#399)

Author: sashabaranov

README.md

@@ -554,4 +554,4 @@ These tests send real network traffic to the OpenAI API and may reach rate limit
 OPENAI_TOKEN=XXX go test -v -tags=integration ./api_integration_test.go
 ```
 
-If `OPENAI_TOKEN` environment variables are not available, integration tests will be skipped.
+If the `OPENAI_TOKEN` environment variable is not available, integration tests will be skipped.