Correct option values to lower case in README.md

  <td>Tuning target for OpenCL driver code</td>

  <td>

  <dl>

    <dt><code>GENERIC</code> (default)</dt>

    <dt><code>INTEL</code> (Intel CPU)</dt>

    <dt><code>PHI</code> (Intel Xeon Phi)</dt>

    <dt><code>FERMI</code> (NVIDIA)</dt>

    <dt><code>KEPLER</code> (NVIDIA)</dt>

    <dt><code>CYPRESS</code> (AMD)</dt>

    <dt><code>generic</code> (default)</dt>

    <dt><code>intel</code> (Intel CPU)</dt>

    <dt><code>phi</code> (Intel Xeon Phi)</dt>

    <dt><code>fermi</code> (NVIDIA)</dt>

    <dt><code>kepler</code> (NVIDIA)</dt>

    <dt><code>cypress</code> (AMD)</dt>

  </dl>

  </td>

</tr>