Add missing end on initial begin in README.md

[danb123]

No description provided.

[yuri-panchul]

The absense of "end" is intentional. In most testbenches this block continues:

initial
begin
    `ifdef __ICARUS__
        // Uncomment the following line
        // to generate a VCD file and analyze it using GTKwave or Surfer

        // $dumpvars;
    `endif

    @ (negedge rst);

    repeat (100)
    begin
        // If probability is 50%, then b is always 1
        a <= $urandom_range (0, 100) < 30;
        @ (posedge clk);
    end