device property: Fix documentation for *_match_string() APIs

The returned value on success is an index of the matching string,
starting from 0. Reflect this in the documentation.

Fixes: 3f5c8d31 ("device property: Add fwnode_property_match_string()")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>