Fix env var docs (#813)

Author: angus-langchain

docs/observability/how_to_guides/toubleshooting_variable_caching.mdx

@@ -13,12 +13,12 @@ If you’re not seeing traces in your tracing project or notice traces logged to
 First, check that the environment variables are set correctly by running:
 
 ```python
-from langsmith import utils
+import os
 
-print(utils.get_env_var("LANGSMITH_PROJECT"))
-print(utils.get_env_var("LANGSMITH_TRACING_V2"))
-print(utils.get_env_var("LANGSMITH_ENDPOINT"))
-print(utils.get_env_var("LANGSMITH_API_KEY"))
+print(os.getenv("LANGSMITH_PROJECT"))
+print(os.getenv("LANGSMITH_TRACING_V2"))
+print(os.getenv("LANGSMITH_ENDPOINT"))
+print(os.getenv("LANGSMITH_API_KEY"))
 ```
 
 If the output does not match what’s defined in your .env file, it’s likely due to environment variable caching.