Fix float expectation example

[benjaminhaeberli]

I saw rominronin reported this error on Discord and you asked a PR to fix it in February, but he didn't make it, so I'm here :)

On writing tests page, the float example is wrong, so I fixed it :

describe('sum', function () {
   it('may sum integers', function () {
       $result = sum(1, 2);
 
       expect($result)->toBe(3);
    });
 
    it('may sum floats', function () {
       $result = sum(1.5, 2.5);
 
-      expect($result)->toBe(4);
+      expect($result)->toBe(4.0);
    });
});