docs(instr-bunyan): fix bug in README example (#1940)

Co-authored-by: Marc Pichler <[email protected]>

plugins/node/opentelemetry-instrumentation-bunyan/README.md

@@ -33,6 +33,7 @@ const sdk = new NodeSDK({
     }),
   ]
 })
+sdk.start();
 
 const bunyan = require('bunyan');
 const logger = bunyan.createLogger({name: 'example'});