Fix ReadStream bufferSize docs

The default is 64kb buffer, not 4kb.

Closes GH-702.

doc/api/fs.markdown

@@ -357,7 +357,8 @@ Returns a new ReadStream object (See `Readable Stream`).
       encoding: null,
       fd: null,
       mode: 0666,
-      bufferSize: 4096 }
+      bufferSize: 64 * 1024
+    }
 
 `options` can include `start` and `end` values to read a range of bytes from
 the file instead of the entire file.  Both `start` and `end` are inclusive and