Remove code tags from INSTALL.txt

The INSTALL.txt document has <code> tags in it (see below), which don't belong in the plugin_reg.php file.

  Add the following lines anywhere above the 'return' statement:

<code>

  $plugins['drupalimage'] = array();
  $plugins['drupalimage']['theme_advanced_buttons1'] = array('drupalimage');
  $plugins['drupalimage']['extended_valid_elements'] = array('img[class|src|border=0|alt|title|width|height|align|name]');