Add "using Interpolations" to knots docstring

I thought it wasn't an issue, because running docs/make.jl would error
on the first time but not the 2nd.

But this just doesn't error

Author: awadell1

src/iterate.jl

@@ -97,14 +97,18 @@ infinite sequence of knots.
 
 # Example
 ```jldoctest
-julia> etp = LinearInterpolation([1.0, 1.2, 2.3, 3.0], rand(4); extrapolation_bc=Periodic())
+julia> using Interpolations;
+
+julia> etp = LinearInterpolation([1.0, 1.2, 2.3, 3.0], rand(4); extrapolation_bc=Periodic());
+
 julia> Iterators.take(knots(etp), 5) |> collect
 5-element Array{Float64,1}:
  1.0
  1.2
  2.3
  3.0
  3.2
+
 ```
 """
 function knots(itp::AbstractInterpolation)