doc: minor argument formatting in stream.markdown

Fixes: #4350
PR-URL: #6016
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>
Reviewed-By: Roman Klauke <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: jasnell

doc/api/stream.markdown

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