Regen docs after POT/PO updates

docs/ja_JP.UTF-8/html/EXTENDING.html

@@ -307,7 +307,7 @@
           The file has the same name as the PNG image, but a "<code>.dat</code>" extension. (e.g., "<code>brush.png</code>"'s data file is the text file "<code>brush.dat</code>", found in the same directory.)        </p>
 
         <dl>
-          <dt><strong>Brush Spacing</strong></dt>
+          <dt><strong>ブラシの描画間隔</strong></dt>
           <dd>
             <p>
               As of Tux Paint version 0.9.16, you can now specify the spacing for brushes (that is, how often they are drawn). By default, the spacing will be the brush's height, divided by 4.            </p>