update otlp-grpc trace exporter class in README.md (#1446)

- was renamed at 113e513

Co-authored-by: Robert <robertlaurin@users.noreply.github.com>
Co-authored-by: Francis Bogsanyi <francis.bogsanyi@shopify.com>

exporter/otlp-grpc/README.md

@@ -35,7 +35,7 @@ Then, configure the SDK to use the OTLP GRPC exporter as a span processor, and u
 require 'opentelemetry/sdk'
 require 'opentelemetry/exporter/otlp/grpc'
 
-exporter = OpenTelemetry::Exporter::OTLP::GRPC::Exporter.new
+exporter = OpenTelemetry::Exporter::OTLP::GRPC::TraceExporter.new
 span_processor = OpenTelemetry::SDK::Trace::Export::BatchSpanProcessor.new(exporter)
 
 OpenTelemetry::SDK.configure do |c|