fix(types): correct type for default export

[harelmo]

Using the example code from the readme:

import retry from 'retry-assert';

results in:

    TypeError: (0 , retry_assert_1.default) is not a function

changing it to:

import * as retry from 'retry-assert';

results in TS throwing the following error:

This expression is not callable.
  Type 'typeof import("/Users/.../node_modules/retry-assert/retry-assert")' has no call signatures. ts(2349)

From what I understand, since this is a CommonJS default export, the d.ts signature should be export = retry.

[harelmo]

@ncjones care to take a look please? 🙏

[ncjones]

@harelmo Apologies I missed your PR. Thanks for submitting this.