bug in CH4/calculator_moore.sv

A bug in the author's Moore state machine program: undefined **EQUAL** state, causing the program to remain in **WAIT4BUTTON** state and crash.
add after line 61 
```systemverilog
last_op[Center]: state<=EQUAL;
```
add after line 80 
```systemverilog
EQUAL: begin
  last_op<=op_store; // Store our last operation
  state<=WAIT4BUTTON;
end
``` 

