Fix installation process steps (#443)

I had to do this to avoid the error message
  Pkg.update ERROR: UndefVarError: Pkg not defined
which was resolved by 
  julia> using Pkg
beforehand.

Author: tcveatch

docs/src/installation.md

@@ -4,6 +4,7 @@ Installation
 Installing Convex.jl is a one step process. Open up Julia and type :
 
 ```julia
+using Pkg
 Pkg.update()
 Pkg.add("Convex")
 ```