update README.md to be copy-able (#13)

sorry for the small PR, but having a code copy-paste error is kind of disturbing.

Author: filchristou

README.md

@@ -25,7 +25,7 @@ pkg> add Multigraphs
 ## Examples
 
 ```julia
-using Graphs, Multigraphs
+julia> using Graphs, Multigraphs
 
 # create a undirected multigraph with 3 vertices and 0 multiple edges
 # use DiMultigraph for directed multigraphs