OTEL configuration documentation is misleading

Hello! 

While implementing the OTEL configuration to be used with our proxy server, i hit a misleading road while setting the endpoint to be used.

Current documentation states that `OTEL_EXPORTER_ENDPOINT` must be setup, when in reality it is named `OTEL_EXPORTER_OTLP_ENDPOINT`

https://opentelemetry.io/docs/languages/sdk-configuration/otlp-exporter/