FFI: Fix docs about C bool -> Lua boolean conversion.

Thanks to M. Kottman.

doc/ext_ffi_semantics.html

@@ -245,7 +245,7 @@ constant values; retrieving return values from C calls:
 <tr class="odd separate">
 <td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">&rarr; <tt>double</tt></td><td class="convout">number</td></tr>
 <tr class="even separate">
-<td class="convin"><tt>bool</tt></td><td class="convop">0 &rarr; <tt>true</tt>, otherwise <tt>false</tt></td><td class="convout">boolean</td></tr>
+<td class="convin"><tt>bool</tt></td><td class="convop">0 &rarr; <tt>false</tt>, otherwise <tt>true</tt></td><td class="convout">boolean</td></tr>
 <tr class="odd separate">
 <td class="convin">Complex number</td><td class="convop">boxed value</td><td class="convout">complex cdata</td></tr>
 <tr class="even">