doc: fix doc for Buffer.readInt32LE()

Update example of readInt32LE method. buf.readInt32LE(1) is supposed to
throw an error as it has only four elements and it tries to read 32
bits from three bytes.

Fixes: https://github.com/nodejs/node/issues/5889
PR-URL: https://github.com/nodejs/node/pull/5890


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>