Fix docs error..

Author: DylanRussell

exporter/opentelemetry-exporter-otlp-proto-grpc/src/opentelemetry/exporter/otlp/proto/grpc/__init__.py

@@ -29,7 +29,7 @@
 - :envvar:`OTEL_EXPORTER_OTLP_TRACES_HEADERS`
 - :envvar:`OTEL_EXPORTER_OTLP_TRACES_ENDPOINT`
 - :envvar:`OTEL_EXPORTER_OTLP_TRACES_COMPRESSION`
-- :envvar:`OTEL_PYTHON_EXPORTER_OTLP_HTTP_TRACES_CREDENTIAL_PROVIDER`
+- :envvar:`_OTEL_PYTHON_EXPORTER_OTLP_HTTP_TRACES_CREDENTIAL_PROVIDER`
 - :envvar:`OTEL_EXPORTER_OTLP_TRACES_CERTIFICATE`
 - :envvar:`OTEL_EXPORTER_OTLP_TIMEOUT`
 - :envvar:`OTEL_EXPORTER_OTLP_PROTOCOL`