fix doctest warnings in src/sage/plot/graphics.py

Author: dcoudert

src/sage/plot/graphics.py

@@ -1816,7 +1816,7 @@ def show(self, **kwds):
         some examples.::
 
             sage: G = list_plot([10**i for i in range(10)])                         # long time, needs sage.symbolic
-            sage: G.show(scale='semilogy')                                          # long time
+            sage: G.show(scale='semilogy')                                          # long time, needs sage.symbolic
 
         ::