doc: minor argument formatting in stream.markdown

Fixes: https://github.com/nodejs/node/issues/4350
PR-URL: https://github.com/nodejs/node/pull/6016
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/stream.markdown

@@ -201,7 +201,7 @@ readable.on('end', () => {
 
 #### Event: 'error'
 
-* {Error Object}
+* {Error}
 
 Emitted if there was an error receiving data.