cmake/README.md: fix GPU_ARCH options

  <td>CUDA SM architecture targeted by GPU package</td>

  <td>

  <dl>

    <dt><code>sm20</code> (Fermi)</dt>

    <dt><code>sm30</code> (Kepler)</dt>

    <dt><code>sm50</code> (Maxwell)</dt>

    <dt><code>sm60</code> (Pascal)</dt>

    <dt><code>sm70</code> (Volta)</dt>

    <dt><code>sm_20</code> (Fermi)</dt>

    <dt><code>sm_30</code> (Kepler)</dt>

    <dt><code>sm_50</code> (Maxwell)</dt>

    <dt><code>sm_60</code> (Pascal)</dt>

    <dt><code>sm_70</code> (Volta)</dt>

  </dl>

  </td>

</tr>