Rename convex_hull.cpp to avoid conflict with the one from Triangulation

Envelope_2/doc/Envelope_2/Envelope_2.txt

@@ -180,7 +180,7 @@ with the same line as the rightmost edge of the maximization diagram,
 and vice-verse. We can therefore skip the rightmost edges of both
 diagrams.
 
-\cgalExample{Envelope_2/convex_hull.cpp}
+\cgalExample{Envelope_2/convex_hull_2.cpp}
 
 \cgalFigureBegin{env2_figex_circ,ex_circle.png}
 A set of four circles, as constructed in \ref Envelope_2/envelope_circles.cpp. The lower envelope and the upper envelope are shown using thick dashed lines of different colors respectively.

Envelope_2/doc/Envelope_2/examples.txt

@@ -1,5 +1,5 @@
 /*!
-\example Envelope_2/convex_hull.cpp
+\example Envelope_2/convex_hull_2.cpp
 \example Envelope_2/envelope_circles.cpp
 \example Envelope_2/envelope_segments.cpp
 */