MathSupport.cpp: fix drawing of leftrightarrow

- to see the difference use this in a formula:
\overleftrightarrow{aasasasasassdsdsdasdsdsd}

src/mathed/MathSupport.cpp

@@ -138,7 +138,7 @@ double const udarrow[] = {
 	0.015, 0.25,  0.5, 0.05, 0.95, 0.25,
 	2, 3,
 	0.015, 0.75,  0.5, 0.95, 0.95, 0.75,
-	1, 0.5, 0.2,  0.5, 0.8,
+	1, 0.5, 0.1,  0.5, 0.9,
 	0
 };