Metaprogramming doc update (minor).

There seems to have been a small change in `Expr(...)` syntax not updated in the manual.

```julia
julia> a = 1
1

julia> dump(Expr(:call, {:+,a,:b}, Any))  # Syntax in manual
Expr 
  head: Symbol call
  args: Array(Any,(2,))
    1: Array(Any,(3,))
      1: Symbol +
      2: Int64 
      3: Symbol b
    2: Any
  typ: Any

julia> dump(Expr(:call, :+,a,:b))   # Actual desired result in manual
Expr 
  head: Symbol call
  args: Array(Any,(3,))
    1: Symbol +
    2: Int64 
    3: Symbol b
  typ: Any
```