Fix typo (invert x and y)

Polygon/doc_tex/Polygon_ref/Polygon_2.tex

@@ -185,19 +185,19 @@ circulators and iterators that allow to traverse the vertices and edges.
       
   \ccMethod{Vertex_iterator left_vertex();}
     { Returns the leftmost vertex of the polygon \ccVar\ with the smallest
-     \ccStyle{y}-coordinate. }
+     \ccStyle{x}-coordinate. }
     
   \ccMethod{Vertex_iterator right_vertex();}
     { Returns the rightmost vertex of the polygon \ccVar\ with the largest
-     \ccStyle{y}-coordinate. }
+     \ccStyle{x}-coordinate. }
     
   \ccMethod{Vertex_iterator top_vertex();}
     { Returns topmost vertex of the polygon \ccVar\ with the largest
-     \ccStyle{x}-coordinate. }
+     \ccStyle{y}-coordinate. }
     
   \ccMethod{Vertex_iterator bottom_vertex();}
     { Returns the bottommost vertex of the polygon \ccVar\ with the smallest
-     \ccStyle{x}-coordinate. }
+     \ccStyle{y}-coordinate. }
     
 For convenience we provide the following Boolean functions:    
 \ccThree{Segment_2}{}{\hspace*{10cm}}