Change first example for sparse arrays

[szarnyasg]

Currently, the first example in the SparseArrays docs is the following:

julia> A = sparse([1, 2, 3], [1, 2, 3], [0, 2, 0])
3×3 SparseMatrixCSC{Int64,Int64} with 3 stored entries:
  [1, 1]  =  0
  [2, 2]  =  2
  [3, 3]  =  0

julia> dropzeros(A)
3×3 SparseMatrixCSC{Int64,Int64} with 1 stored entry:
  [2, 2]  =  2

This example is shown before the COO format is explained (and it needs to use COO as it deals with explicitly setting zero values). If a user scans this page superficially - e.g. when looking for a quick example that shows how to initialize a sparse matrix -, they might be inclined to think that the function

sparse([1, 2, 3], [1, 2, 3], [0, 2, 0])

creates a 3x3 matrix of the values listed in the arrays and that way confuse it with

sparse([1 2 3; 1 2 3; 0 2 0])

(The latter syntax is also there on the page, coming from the docstring of findnz).

I have been confused by this multiple times when I forgot the syntax and visited the Sparse Arrays documentation page to refresh my memory. To mitigate this, this PR proposes a simple change and adds an additional non-zero entry, which hopefully makes the docs a bit more instructive.