Fix formatValue() reference

[novemberborn]

Fixes #1321.

Turns out we don't have any real integration tests for assertion enhancement. I don't have time to add those right now either, but at least this fixes master.

With the sample code from #1321:

❯ "$(npm bin)"/ava test.js

  1 failed

  [anonymous]
  /private/var/folders/_6/p8qxp_3n62zg9081tvb0lcc80000gn/T/tmp.okeY2IXLOl/test.js:5

   4:   const x = () => 'foo';
   5:   t.regex(x(), /\d/);
   6: });

  Value must match expression:

    "foo"

  Regular expression:

    /\d/

  x()
  => "foo"

[sindresorhus]

👍

I've opened an issue about better integration tests: #1324