plug-ins: Fix typo in gfig-arc

Resolves #5742.
The check is for a horizontal line (where the Y coordinate
would be the same for all three points), but the X axis was
checked instead due to a copy/paste error.

plug-ins/gfig/gfig-arc.c

@@ -270,7 +270,7 @@ arc_details (GdkPoint *vert_a,
   if (ay == by || by == cy || ay == cy)
     {
       /* Horz line -> midpoint gives inter_y */
-      if (ax == bx && bx == cx)
+      if (ay == by && by == cy)
         {
           /* Straight line */
           double minx = ax;