fix PID module

Author: gfwilliams

modules/PID.js

@@ -43,4 +43,4 @@ PID.prototype.step = function(err) {
   return this.out;
 };
 
-
+exports = PID;

modules/PID.md

@@ -25,6 +25,7 @@ response you'll want to call `pid.step` at a set time interval.
 For example to modulate temperature for a heater:
 
 ```JS
+const PID = require("PID");
 var pid = new PID({
   Pg : 1, // proportional gain
   Ig : 1, // integral gain