Fix contrib/seg's GiST picksplit method.

Fix the same size_alpha versus size_beta typo that was recently fixed
in contrib/cube.  Noted by Alexander Korotkov.

Back-patch to all supported branches (there is a more invasive fix in
HEAD).

contrib/seg/seg.c

@@ -432,7 +432,7 @@ gseg_picksplit(GistEntryVector *entryvec,
 		else
 		{
 			datum_r = union_dr;
-			size_r = size_alpha;
+			size_r = size_beta;
 			*right++ = i;
 			v->spl_nright++;
 		}