doc: fix buf.readInt16LE output

Previous example didn't account for the 1 byte offset

PR-URL: #5282
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Roman Reiss <[email protected]>

Author: chinedufn

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])