Fix otlphttp endpoint in docs

[dawhalen]

The otlphttp endpoint described in the docs here is incorrect, if you attempt to use an OTEL Collector with this endpoint, you'll get this error from your collector:
2025-07-10T13:44:01.837Z error internal/queue_sender.go:103 Exporting failed. Dropping data. {"kind": "exporter", "data_type": "traces", "name": "otlphttp/langsmith", "error": "not retryable error: Permanent error: rpc error: code = Unimplemented desc = error exporting items, request to [https://api.smith.langchain.com:443/otel/v1/traces/v1/traces](https://api.smith.langchain.com/otel/v1/traces/v1/traces) responded with HTTP Status Code 404", "dropped_items": 1}

The collector automatically adds the /v1/traces for you, so it should not be included in the collector's endpoint config.

[vercel]

@dawhalen is attempting to deploy a commit to the LangChain Team on Vercel.

A member of the Team first needs to authorize it.