doc: fix buf.readInt16LE output

Previous example didn't account for the 1 byte offset

PR-URL: https://github.com/nodejs/node/pull/5282
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/buffer.markdown

@@ -728,7 +728,7 @@ const buf = new Buffer([1,-2,3,4]);
 buf.readInt16BE();
   // returns 510
 buf.readInt16LE(1);
-  // returns -511
+  // returns 1022
 ```
 
 ### buf.readInt32BE(offset[, noAssert])