doc: remove an unused arg in process.stdin.

The argument of process.stdin's readable event handler is not used.

doc/api/process.markdown

@@ -170,7 +170,7 @@ Example of opening standard input and listening for both events:
 
     process.stdin.setEncoding('utf8');
 
-    process.stdin.on('readable', function(chunk) {
+    process.stdin.on('readable', function() {
       var chunk = process.stdin.read();
       if (chunk !== null) {
         process.stdout.write('data: ' + chunk);